Except it's not free to use without the OGL. If you're going to make derivative work from the SRD or other Open Game Content, you have to use the Open Game License have free use and avoid possible legal complications.
The OGL is there to acknowledge who produced the original content you're intending to base your own work on, and it's only fair to follow the OGL if you're going to produce derivative work. It doesn't matter that you're not planning to profit off of it, the point is that it's probably not 'fair use' if you don't comply with the OGL and include an OGC declaration.
You can't use terminology or specific lines of text (or specific tables or the like) out of the SRD unless you adhere to the Open Game License. If you rewrite all your work to avoid that and to avoid copyright infringement, then it's likely people will not be able to understand whatever it is you had written, and it will have been a pointless effort to put the material out there.
The game mechanics can't be copyrighted, but the actual text describing how they work, and the names for those mechanics, are copyright material (and the D&D name itself is trademarked, which is why other products these days don't say they're compatible with Dungeons & Dragons, they instead use a substitute like 'the third edition of the world's most popular roleplaying game'). You would have to go to some significant lengths to write everything in different terms and using differently-formatted tables, if you were going to avoid copyright infringement while also avoiding the OGL for some odd reason.