Wow, splendid explanation. Here's the scan for a better illustration:
I had too forgotten that Wolverine literally commented about their resemblance --I guess I believed it was all in my mind because bias.
Nevertheless, I never thought that Gambit was a transformed Longshot, no way. How could that be? For starters, Longshot never crossed the Siege Perilous --he simply said he must go out to find himself and his exit was off panel (he probably went back to his own dimension with Gateway's help).
When I first saw Gambit my exact thought was 'Why, it was about time. It was not
so difficult to create a character like this!'. Regarding the similarities (spikes, glowing eye), well, I'm afraid I simply thought that it was a matter of lack of originality. Traditionally, sneaky characters have been portrayed this way: 'glow in the eye' is a graphic conventionalism for slyness and malice --I never took this as a sign of 'luck power'. As for the spikes, they're equally a quite treacherous weapon of choice. These two traits, though, work in a very very very different way for Longshot.
Fortunately, Gambit gained soon a personality of his own. Cards were certainly a touch of genius.
Here the former Gambit as Mike Collins first depicted him, based in Jim Lee's concept.