What happens when u buy Sterak's Gage + Maw of Malmortius?

Both are mutually exclusive. Their shield passives share the same name so they do not work together. Only one will activate, depending on which criteria is met first, and then both will share a cooldown.

Both I think, because one is a shield and one is magic damage shield.

(I could be wrong but thats my educated guess.)

Each trigger is separate and the shields can overlap, magic damage will hit Maw first.