Update: 50% off coupon is dead. If you're still interested, here's an intended 25% off code: DEALZO-N25OFF-STAGMG
For those aware we usually post game/hardware deals on main front page feed but since this is probably a price mistake, we'll keep it in the forums (and on the down low) for now.
GMG rarely makes price mistake (they do honor them) but not to this magnitude.. so we're not sure if they'll cancel these orders or not. Mad Max orders should be safe as this code was INTENDED for Mad Max only, but not the other deals.
These are of course pre-orders and keys will be delivered near or on release date (usually if there's pre-load, GMG will send out keys early depending on publishers).
Only if they developed a version for other OS' and steam has it as a steamplay title (buy once play on all platforms) But this wont happen for Fallout 4 as its windows only.
Yes the steam logo (below) signifies that it has support for Steam OS and GNU+Linux
The gear icon (below) just signifies that it uses the steam client. and the 'steamplay' icon signifies that you can play it on multiple platforms, but not necessarily all platforms, it will have a logo for each one it supports.