The difference between altcoins and gem coins is simple, altcoins is any coin in the market that isn't bitcoin, no matter their use case or whatever they are all regarded as altcoins, while gem coins is usually used to refer to altcoins that has so much potential or room to grow, gem coins are rare to find and sometimes you only hear of them after they have pumped over 100x, there's no specific place to find gem coins you just have to do your research before investing in any project and pray that it will do well in the future.