[Freeciv-Dev] What proof is needed that a given piece of code is GPL?

[sam+civ@xxxxxxxxxxxxxxxxxxxx]

Hello there,

I have found some useful code on the internet which I would like
to integrate into FreeCiv (some code which simulate plate tectonics
to generate random maps).  The code, as it exists on the internet,
is under a non-GPL license.

However, I have obtained permission from the author to re-license
this code under the GPL.  What proof do I need to present to the
FreeCiv development team that this code now has a GPL license, and
can therefore be legally added to the FreeCiv codebase?

- Sam