I do not believe that a GFI is required. Your concerns as to motors vs. GFI's are probably not as justified as they once were; the newer (made in past year) GFI's are supposed to be re-designed to reduce such "nuisance tripping' problems.
As I understand your application, the compressor only supplies air pressure; it is the air pressure that operates switches. In that case, you already have "removed" electrical service from the 'at risk' area, and adding a GFI -while not harmful- is, in my opinion, completely unnecessary.
