yep, i think Undertale is where i first heard of Gamemaker. you can find a list of major games made with GM on YoYo's website if you weren't aware of it already. lots of cool ones that you might recognize.
For me it was Spelunky that I first learned was made in GM. It was really weird because when we had substitutes in my 7th grade Game Design class (where we were using GM8), we would play Spelunky. But I didn't learn until 2 years later that it was actually made in GM.