for my understanding, TheGameCreators were the only ones, who never limited the assets to the engines. Because they had more products in there portfolio. e.g. Dark Basic and FPSC, or some tools, and converters.
In some stores the assets a limited to the engine, like most of the assets from RPG Maker are only for use in RPG Makers, also music and so on. And CryTek-Assets are also with restrictions.
I am not a lawer, but from what I understand, how TheGameCreators promote there Tools and Assets, it is exactly intended to use your assets from one product, also in the other.
An if you are not sure, you can only use these assets:
https://github.com/LeeBamberTGC/FPS-Creator-Classic
There is also an FPSC-2-AGK Tool available, and now GameGuru-Loader, so that you could have this tool-chain: Asset -> FPSC -> FPSC Reloaded / GameGuro -> AGK2