Hello! I'm wondering, if I'm using OpenTNL in my closed source project and I'm not going to get any money from it, I will give it away for free, will I be able to do that? Or do I really have to buy a license?

This should probably be in the business of game development.

In general, GPL doesn't care if you make money or not. If you link with GPL code, you have to make your code GPL, too.

It doesn't matter if you plan to make money or not. If you don't wish to open the source to your application, then you must buy a closed source license for OpenTNL. If you use OpenTNL without buying a license, then you must abide by the terms of the GPL and open your source. There's no gray area here.