...Of course Thumbs would have a page on Splatoon 2 why didn't I think of this.
Don't worry about gear grinding until you hit ~lvl 30; it costs a crazy amount of squid cash which for now would be better used buying all the weapons and gear you can. But, when you do get there, here's a
Generalized SPLATOON 2 Gear Guide (based on limited data mining info)
Each piece of gear has one main ability, 1-3 sub-ability slots, and a brand. The main ability can (almost) never be changed. The brand can absolutely never change. The sub-abilities fill in as you the level the gear. A gear's brand influences your likeliness to roll specific abilities in your sub slots; each brand has 1 ability it's 5x more likely to roll, and another it's half as likely to roll. Special gear (like from the story mode or amiibos) are equally likely to roll anything. Gear can be bought in the in-game store, the Splatnet online store (more on this later), or ordered from the inklings walking around your plaza (ex, if you like someone's hat you can buy a copy of it for an upcharge). Crusty Sean sells drinks that temporarily improve your likeliness of rolling specific sub-abilities, as well. These can only be bought using specific tickets that you randomly get from Salmon Run rewards.
A main ability is a little more than 3x as effective as a sub ability of the same type. Abilities of the same type have diminishing returns (we're still working on the math). If you go to the ability status pane of the menu, you can see all the abilities in the game. Each ability that doesn't have a # of chunks beside it (the bottom 12) are special. These can only be main abilities, never rolled as subs, and are exclusive to different categories of gear.
Opening Gambit, Last-Ditch Effort, Tenacity, and Comeback are exclusive to Hats.
Ninja Squid, Haunt, Thermal Ink, and Respawn Punisher are exclusive to Clothes (Ability Doubler currently doesn't exist in the game ircc).
Stealth Jump, Object Shredder, and Drop Roller are exclusive to Shoes.
Abilities, generally speaking, have very minimal effects. Stat information is still coming in, but expect most abilities to be almost unnoticeable during play. This guy's channel has plenty of comparison videos from the first game, and things don't look drastically different so far.
If you're looking to optimize your gear, there are two currencies you need to care about: ability chunks and super sea snails.
Snails are used to increase the number of sub-ability slots on 1-2 star gear, and to reroll all three sub ability slots on a fully leveled piece of gear (like a slot machine). You also get one ability chunks for each sub-ability you replace. Unlike the last game, it looks like there's no way to purchase snails with money, meaning you can only acquire them through splatfest rewards and leveling up (you get one at 30, and one more each level after that). Given their new scarcity, I'd advise you only use snails to add ability slots to gear since there are certain combinations of abilities and brands that only naturally exist on 1 and 2 slot gear.
Ability Chunks are used to put the ability of your choosing into an empty sub ability slot. It normally takes 10 chunks per slot, but gets more expensive when applying identical sub abilities to the same piece of gear (ex, the 1st run speed up sub costs 10 chunks, the 2nd 20, and the 3rd 30, so it would cost 60 chunks to give a piece of gear 3 identical subs). You get chunks through scrubbing gear, rerolling gear, and replacing gear.
Scrubbing empties a gear's sub slots, gives you one chunk for each ability removed, and costs $20,000. Unfortunately, it looks like you can't scrub specific slots; you have to remove them all. If you would ever receive a piece of gear you already have (say, through ordering something, or getting another copy of this month's Salmon Run shirt) you have the option to keep your existing gear or keep the new gear and turn all of the old gear's sub-abilities into chunks.
So what's should you actually be doing?
First, wait until all the statistics come in and 'the meta' coheres. We honestly don't know which abilities are good for what yet, even if we have suspicions (ex, ink resistance is great for mid-close range weapons, Quick Respawn is terrible). It takes so long to make perfect gear that you don't want to waste any resources. When you start grinding, your goal will almost always be to give your gear 3 identical sub abilities. It'll cost more chunks, but will give you more flexibility to mix-and-match your outfit. Pick your gear based on its main ability and what sub its brand favors, then max out its sub slots with snails if necessary. This spreadsheet has information on every piece of gear you can buy in the in-game store, and will be invaluable when planning your build.
Oddly enough, for the moment it looks like the optimal way to grind ability chunks is through Salmon Run. You get random chunk rewards at a far faster rate than fully leveling 3 star gear + paying $20k, even if you're less likely to get the specific chunks you need. Once you get a drink ticket for the ability that matches your gear's brand's preference, then you should scrub your slots and start releveling your gear. At that point, you're reasonably likely to just naturally roll 3 of the same sub ability, chunks be damned.
The Giant Asterisk
There's one new feature that throws a pretty complicated wrench into planning out your build: Splatnet Gear. If you download the Nintendo Switch app (the terrible thing everyone makes fun of for its abysmal voice chat integration), there's a Splatoon 2 shop that lets you order secret variant gear. Gear ordered through this page (and picked up from Murch, the spiny kid) has a different a main ability. The inventory changes every couple of hours and items are put up in seemingly random combinations. This is a big deal for many reasons:
Assuming the ability combinations are in fact random, then any Main+Sub ability combination in any gear slot becomes realistically possible, if very rare.
Getting this variant gear replaces the non-variant version. Ex, you buy a pair of Blueberry Casuals online with Special Saver as its main ability. Normally, those shoes would have Ink Saver (Sub). Now, if those shoes show up in the in-game shop, you can't 're-buy' them to get the normal version back. You're stuck with Special Saver unless you manage to find someone wearing the normal pair in your plaza. This means you can accidentally screw yourself out of good gear by buying worse variants of it in the online shop. If you're going to buy anything from the Switch app, check that spreadsheet I linked above and compare it to its normal version. Do you like that ability better? Are there other pieces of gear with the same main+brand combination? Etc.
Hope this is helpful to everyone.