When I started making a game demo I found that I needed to draw a circle and a mask function.

Does DragonRuby Game Toolki you have these features? If not, what advice do you have? Thank you.

Do you have a gif of what you're wanting to do?

Masking is definitely needed (and currently not supported). But with a sample, I might be able to Jury Rig something for you until we add that capability to the engine.