Probably a Mystic Adept would be best for this.
This will give you Spell casting abilities which can replicate his power to convert matter into energy projectile's. Power bolt would seem an appropriate choice. You can also make it a limited spell so it can only be cast with a Fetish, such as a pack of cards. This will make the Drain of the spell easier to resist and obviously match Gambit's use of cards.
The remaining Power Points can be spent on Physical Adept powers which can be used to create his amazing athletic abilities.
You might also consider taking a Mentor Spirit, the Trickster seems entirely appropriate