How do you make custom gims in gimkit?

In the actual website, you can't. However, if you want to make one as a model in gimkit creative, though, you can. First, take a sentry and use it as a placeholder. Then, stretch shapes around it in any color as long as they're the same size as the sentry. Then delete the placeholder.

Why aren't custom shapes collidable?

They are off by default. You can't make them collidable.

What are those solid color, neat shaped blocks?

Those are Barriers in the Devices section.

Who is gimkit's biggest rival?

Blooket