I tested this week to play from Linux and it worked, but I did not use any client but downloaded from the website and uploaded the same way

The Client and the website are actually scheduled to be replaced. But currently this is a sideproject I believe.
But the new GMR (at least the client portion) is open-source . Look for the new GMR Thread here especially the Mac/Linux Thread. Currently development is in the very early stage for it as they started over . But the windows client should share the same codebase now.
Whats the status of the new website I have no Idea though.

Interesting I wanted to try the Linuxversion in a week whne I'm traveling. I guess that is something for Nullmaster to look at

If that indeed happened then that would be a bug in the game , so for Firaxis. I don't think someone here can really debug Civ itself

Yeah

Do you have another diplomatic offer open. You can have only one offer open at the same time.

if you use a mod its difficult as they are not included . But the real pick is in Game anyway . In GMR you only pick for Display purposes of the website. If you have agreed who plays what then the player setting up the game in Civilizstion does it when creating the game.

Its mainly that the current version is just kept together while the new one is developing - unfortunately its a bit delayed.

Crossplattformtools I don't know. For Linux you could look at the openSUSE Build Service (OBS) which allows you to build packages for a variety of different distributions. (https://build.opensuse.org/) not sure if that is what you are looking for .