This problem is caused by Steam, not GameAP.
You should upgrade SteamCMD of GameAP like this:
Delete or rename this folder: /srv/gameap/steamcmd
sudo dpkg --add-architecture i386
sudo apt install lib32gcc1
mkdir /srv/gameap/steamcmd && cd /srv/gameap/steamcmd
curl -sqL "https://steamcdn-a.akamaihd.net/client/installer/steamcmd_linux.tar.gz" | tar zxvf -
mkdir /srv/gameap/servers/steam
find /srv/gameap/servers/steam -type d -exec chown -R gameap:gameap {} \;
Then try to install server again! With this latest official version of SteamCMD you will get error by less chance.
Unfortunately you can still can get this error but I think it is a bug of SteamCMD.
Keep pushing install till it works!
If you get one server install you can make a copy of it to your others, then just tick it "installed"!
