What will they be like in pvp

Dragon Tail/Outrage is pretty bad in PvP compared to something like Dragon Breath/Dragon Claw. However, Mudshot + Earthquake and Fire Blast are pretty good and also provides some nice coverage. It also has more bulk natural bulk and a resistance to Rock and a triple resistance to electric will be very useful. I think it should be fairly good. Not gonna say anymore since it's not out yet.

I think Garchomp will be better for ground offense than dragon offense. DT/OR just doesn't work as well in PvP, you'd be better off using Giratina, Latios, or double legacy Dragonite.

MS/EQ will give you a non-legendary alternative to Groudon with a different set of resistances and weaknesses, allowing for different team compositions. Outrage as a second move instead of Fire Blast keeps it from getting walled by Dragonite.

Probopass will probably be good in Great League. Magnezone's charge moves don't give me a lot of confidence in its PvP abilities.

Probopass is going to be extremely good, pretty much a better Melmetal in great league.

Garchomp probably isn't horrible with MS/OR/EQ, but I don't see what it does in lower leagues that MS/DC/EQ Flygon doesn't do better. In Masters it's probably more like an alternative to Groudon just like BiggestLeaf said.
Gabite looks mildly interesting for great league, it's a dragon that doesn't get walled by Skarmory thanks to packing Flamethrower as a secondary move. Too bad both of its STAB charge attacks kinda suck.

And Magnezone seems decent like I wrote already. S/WC is fairly spammy and hurts a lot.

Garchomp (like the other pseudo legends) has high attack stat, so it will be hindered by the CP limits in great and ultra leagues. If it is going to be good in PVP, it will be in master league.