I agree with everything stated above, except I think lunar cancelling is too inconsistent, and it has very limited uses even if you could do it every time. Forget about lunar cancelling. Flame Pillar is also pretty laggy, so I tend to use it sparingly. Gundashing is a great escape tool, but do you really want to be one of those players who always runs away from pressure? Those players always lose. Even with limited tools, one must fight at close-range under pressure, it's necessary in this game.
Reflector is essential, in my opinion, when recovering/landing in conjunction with gundashing, or when the opponent is comboing you or trying to stay very close/behind you. Particularly against Mario, Bayonetta, Pikachu and others like this, reflector can stop a combo just like Yoshi's nair. Without reflector, I feel like easy prey as Gunner.
Another option is fullhop nair and then fair/bair in the same jump while landing. Flama does this a lot, especially against characters like Sheik who aproach with shorthop aerials... your fullhop nair can challenge Sheik's fair approach, and Gunner's fair when landing prevents any followup attempt to punish your nair. This seems to work effectively on any stage, but especially Battlefield, because the 2-layered approach of this attack matches the stage design.
My current favorite mixup is shorthop nairs and RAR bairs, combined with dtilts in either direction. Dtilt is a pretty fast move with a big hitbox, and does like 14 damage, has acceptable lag, and can kill. This makes it superior to most options at close range. If you use lots of shorthop bairs, be sure to practice fastfalling just after the move is finished to avoid landing lag. If you fastfall during a SH bair (or nair) too quickly, you'll get lots of lag, and this is a major reason Gunner might lose to close-range pressure. So practice that, use Jab/Dtilt a lot, and get pro at gundashing (set C-stick to Attack). That's all I can think of so far.