Hi Ellie,
I am not familiar with Gemini - personally, I use Bitbuy, Coinbase is a big one and I've heard good things about Shakepay. There are many options out there, but the two I've listed are the one I am most familiar with. I chose Bitbuy based on recommendations from an expert in the field. It's important to be picky about where you buy Bitcoin as there are almost too many exchanges out there.
Shakepay also seems to be a legitimate option, one that has no fees and seems to be gaining in popularity. Are you on our Discord yet? We are having lots of discussion about cryptos and Bitcoin in particular. Many options have come up, but Bitbuy and Shakepay seem to be the most common. For what its worth, I just did a search and Gemini didn't come up once in those discussions.
Mat