Official Servers: The official Steam servers are largely populated by die-hard fans using specific patches (FAF is for SupCom 1, SupCom 2 has a smaller community). The FitGirl repack is the GOG version, so you cannot play on Steam servers directly.
LAN/Tunngle/Hamachi: This repack supports LAN play. You can use VPN software like Radmin VPN or Hamachi to simulate a LAN network with friends who also have the GOG version.
Version Mismatch: If trying to play multiplayer, all players must have the exact same version. Since this repack is usually the latest GOG version, it works well with other GOG copies but is incompatible with Steam copies without community patches.