someone wrote a nice framework for developing html5 3d engine. It's GPL/MIT licensed. Do you think it's a good idea to build upon it, and then repackage it and sell it as a service?

For eg> the framework lets you build 3d environments. so I build on it, maybe add a few features like characters and music in the environment, and turn it into an online game.

Is it a feasible idea?

Of course, part of the GPL license says that all source code has to be visible to end users.

Assuming the web is a winner take all market based on first mover advantage, why care about hiding the source code? It's the product that's being sold that counts.