There's loads but here's a few:
CND Shellac(not strictly a gel but in the same arena), Gel II is getting fab reviews, Gelish, OPI gelcolor, Jessica Geleration, Cuccio, IBD, Gellux
I'd spend your time and research each one. Don't be solely led by bottle prices, you need to look at cost per service. Each supplier should be able to give you this info. Also look for prices of lamps - normally you are advised to use the same lamp brand as polish and shouldn't mix systems. There are exceptions to this but I'd tread carefully if you choose a lamp that hasn't been manufactured by the same brand.
Then look at colour ranges, ease of removal etc.
There's loads of threads so I'd recommend sticking the kettle on amd doing a search for some of the brands I've mentioned.
Personally I use CND Shellac and am just starting Gel II. I've previously used Gellux
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