Using OTTD to introduce 5th+ to engineering

[Davegb10]

To start off I am a STEM (science technology engineering math) counselor at a non profit club for boys and girls in America. I understand that this is a fully open-sourced reimplementation of TTD, written in C++, but what does that imply from a legal standpoint ive briefly read over the "COPYING" and readme but in no way am I a lawyer. Can I get a short breakdown of why or why not a non profit organization may or may not be able to use this or where to purchase to be legit.

[planetmaker]

To start off I am a STEM (science technology engineering math) counselor at a non profit club for boys and girls in America. I understand that this is a fully open-sourced reimplementation of TTD, written in C++, but what does that imply from a legal standpoint ive briefly read over the "COPYING" and readme but in no way am I a lawyer. Can I get a short breakdown of why or why not a non profit organization may or may not be able to use this or where to purchase to be legit.

Neither am I a lawyer, but how I read the GPL in threefour simple sentences:
* You may download it and use it for whatever purpose you like.
* You may distribute in an unmodified form including documentation and licensing information however and to whomever you like.
* If you modify the source: you may do the same as with unmodified programme under the condition that indicate your changes and that you distribute the complete source code in the same manner as the programme itself and at no additional charge; that condition is considered fulfilled usually, if you offer a download of the complete modified source accessible to the general public .
* There is no warranty whatsoever

[Davegb10]

Thanks also such an amazing job you guys did!