Gym biking, what it will do to my body? It will make my muscles bigger or make my legs thinner?
Unless you don't do it with much resistance, you won't grow big muscles, just tone them.
And of course you will boost your metabolism depending on your effort and thus lose fat, also on the legs of course.
So they may appear thinner and toned which is exactly what you want I guess.
Hey my uncle was a pro bicycle racer once and has trained alot and passed some onto me. It will tone your legs and put muscle in your butt. I've seen these results in everyone he bikes with. Just be consistent and do it at least 3 days a week and be sure to keep your pace or rpm around 90 and do it for at least 10 minutes