Gideon Jura in creature form + combust

[perm]

If Gideon Jura becomes a 6/6 "prevent all damage" creature, If i use a damage prevention bypassing spell like combust, does he lose loyalty? or would damage just be dealt to him in creature form?

[Malchar]

If you activate Gideon Jura's third ability and then unpreventable damage is dealt to him (due to Unstable Footing, for example), that damage has all applicable results: specifically, the damage is marked on Gideon Jura (since he's a creature) and that damage causes that many loyalty counters to be removed from him (since he's a planeswalker). If the total amount of damage marked on Gideon Jura is lethal damage, he's destroyed as a state-based action. If Gideon Jura has no loyalty counters on him, he's put into his owner's graveyard as a state-based action.

Gideon will take damage and lose loyalty. This is explicitly answered in the rulings for the card on gatherer, which is a good place to check if you have any questions.