Aside from americolor (and wilton), what are other good brands of gel food coloring? I'm in the US.

You'll find lots of food colors recommended, but keep in mind that foreign made food colors may not be considered edible in the US.  Many food colors from other countries contain additives that aren't approved by the FDA, or are outright banned for use in the US.  If it uses E or C numbers solely in the ingredients list, or describes the product as "non-toxic", it probably won't pass as edible here.

Other than the 2 that you've listed, I can think of 4 more:  ChefMaster, CK products, Chef Rubber Color Emulsion Gel, and a newcomer TruColor.  I've tried the first 2 and they're fine.  I've used other Chef Rubber products and I like them.  I hear good things about TruColor.

I've been hearing about ProGel lately and how they're highly concentrated so you get deep colors very quickly and the tube will last you much longer. I haven't tried it out myself yet but I'm dying to

ProGel colors are made by Rainbow Dust, a UK company.  Some of the colors contain FDA banned additives.  Rainbow is working on a US FDA compliant line, but it's not yet available.  Yes, you can find the colors online from people who've gotten them into the country, but they are NOT for use on food here.

I have used ProGel & find they have a much wider variety of colors to choose from.  Most of the ones I have tried don't seem to be any more concentrated than the Wilton colors though.

I've been buying ChefMaster colors lately. So far I've only used their black airbrush color to make black dark chocolate buttercream and it's worked perfectly! I stocked up on a bunch of their gel colors (bought them from bake deco.com, they are very affordable!) and honestly haven't used any of them yet so I can't personally attest to how well they work. However, I've heard great things and read very positive reviews, plus they are significantly cheaper than Americolor gel colors which is great.