Does this include Gat Out of Hell?

No, Saints Row Gat Out of Hell was a separate release, it was included in a double pack with SRIV: Re-Elected once but this is not the standard release.

I am in AUS – why can’t I play with my friend in another country?

Saints Row IV: Re-Elected has different content due to ratings in AUS, so you can only play with others on the same version as you.

Is there a way I can stop the upgrade and keep my version of Saints Row IV as it is?

You can disable auto-update on GOG Galaxy if you have the game currently installed. (Cogwheel > Settings > Game features > Untick 'Auto-update games') A Legacy Version of Saints Row IV will be also be available via a separate Beta Branch on GOG and Steam.

To access the old version of Saints Row IV, right click on the game in your library, go to properties > beta and use the dropdown menu to select the version you want. for more help with this: https://help.steampowered.com/en/faqs/view/276C-85A0-C531-AFA3 However the differences between old and new versions will be minimal, and save games will be compatible.

When will the Linux players get their upgrade?

We will update the community when we have a locked in date for this.

Can Linux players crossplay now or do they have to wait?

Official support will come for Linux later – we will let you know when this is going to be available.

Will Mods be affected?

Mods should not be affected, and should work as normal aside from any special cases where elements of the mod may clash with the code of the game.