There is a mage guarding and a deathfang guarding in a zone.
A Gremlin wants to attack the mage, can he do it?
Or can the enemy mage decide, that the deathfang has to be targeted?
Attacker decides.
Guarding just means the attacker has to attack a guard.
"If you are making a melee attack, the target must be in the
same zone as the attacker. If there are enemy guards in the
zone, you must choose one of the guards to attack (see
“Guarding” on page 29)."
"If there is an enemy guard (a creature
with a guard marker) in a zone, then you cannot melee
attack any object in that zone other than enemy guards.
This condition is checked when the attack is declared."