1. Spark
2. Vet
3. Karl
Karl is a Striker with a leaning towards Pusher. He can still jungle effectively (all Strikers can reach the jungle at Pro Level 1). Karl's stun is godly. They nerfed the speed of his Stun and the deployment timer for Juniors, but he's still great to lock down grappled opponents (his stun breaks enemy grapples and can turn friendly grapples into chains so they never escape. Also, he's godly on Loco Moco since he can stack 2-3 Juniors at the jumppad to the annihilator which will practically one shot anyone that comes in range.
My Karl leveling:
1. Junior
2. Offense
3. Skip
4. Junior + Offense
5. Stun
6. Stun
7. Skip
8. Junior + Offense
9. Stun
10. Defense (or Prop Hop if playing Loco Moco)
11. Prop Hop/Defense
12. Defense
13. Prop Hop
14. Defense
15. Prop Hop
I find Gunner to be one of the toughest Pros to play since he's so dependent on heals and he has no escape options or mobility. Unless you're running a pocket Support, Gunners are pretty much Commando/Sharpshooter fodder IMO. Also, while the speed of his rocket was nerfed at low skill levels, it's actually been buffed compared to the early iterations of it. The turn radius used to be horrible and the speed was slower than the current level 4 version, but faster than the current level 1/2 versions. Uber changed the rocket so it has higher speed than it used to at level 3/4, but it also has a much tighter turning radius so it can chase down targets much, much better. Also, the rocket got a knockback buff 2 patches ago (RC2 IIRC) which has enabled him to knock people off the Anni at range and score random ringouts. Only downside to that "buff" is that good players will use the knockback to escape from the gunner.
Also, IMO, leveling rocket last as a Gunner is terrible. Rocket is a huge part of Gunner's DPS and neglecting that is only hurting yourself. Deploy should always be the last skill to level up. The only use it currently has is for a quick 250 armor and to increase reload speed. If you sit in Deploy for more than that, you're just making yourself an easy target for Commandos or anyone with no-falloff weapons. Ex: As Spark, I can switch to my Blaster and destroy 1/3 to 1/2 of a Gunner's health in the time it takes him to Deploy/Undeploy.
Keep your range against him. He should never come within Jetgun range. If he does, Disk him and Shoot the Moon (or Rocket Jump) away. If you have trouble making long-range rockets connect, use the alt-fire on Rocket Ma'am.