I know that with gas the torch should be + and without gas it should be -, but I don't understand why. Can anyone explain the physics behind it?

It's so the puddle melts the wire instead of the wire melting the puddle, flow of electrons and all that technical stuff for metal transfer. On the welder I had if you swapped the earth over it didn't operate the gas solenoid as it didn't need to.

I konw that with my Tig welder when using it for stick welding the polarity must be reversed.

Its all to do with the fact that when stick welding you want the electrode to erode i.e. be the anode. Whereas with Tig you don't want the electrode to erode so the current is reversed.

The electron flow in a DC current is neg to pos that is anode to cathode if I remember my A level physics. Probably changed since I went to school! everything else has.

Prior to gasless migs, fluxed cored wire was used in normal migs and it welded fine without touching the polarity. Gasless migs came later and had the reversed polarity, it must help or they wouldn't do it.

I did a few tests and found no noticable difference using a high quality normal gas mig

to simplfy workpiece to wire temp ballance

you can have many types of cored wires in the pro end of the market for spechal applications and are not always the same polarity but some of thease still use gas

same can be said for mma/stick rods as some can work in both polarity some are ac some are neg or pos to the rod only