Wizkids Rules!

Gint in his Admiral form states;

When a friendly captain would spend one or more GPLT;
You may roll a number of defence dice equal to the number of GPLT that would be spent. For each evade result spend one fewer GPLT.

Is this a once per turn ability as it is a 'may' effect or can each Friendly captain use this ability every turn?

This falls under the "may" rule and is once per turn as a result.