Searching For That Costume?
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I understand. My son does it to me every year. My site stats tell me that someone was searching for "gambit costume pattern". Here's how I did it 2 years ago:

First I searched google images for a good pic of Gambit. I printed the pic (read: stole it off the internet... Fair use?) Then I browsed the patterns at a local sewing shop. I chose a knock off Matrix (first movie) pattern for women. Partly because the men's sizes were too large, and partly because I needed the extra chest room to accomodate the breast plate. I bought some cheap tan upholstery fabric while I was there. Now, Gambit's coat collar is larger than the pattern's collar, so after I cut out the pattern, I sorted through the scraps and found the perfect collar pieces. I think they were originally the armpit bits. I used the smallest size (6), but gave it a size 8 waist. Very easy to do. Just measure your child and apply the measurements to the pattern. The upholstery fabric gave the coat a little stiffness, which was perfect.
I couldn't have done it without Persephone. She did all the actual sewing.

Next came the tough part... the breastplate. I bought some 34 gauge copper sheeting (it comes in a tube at craft stores) and tried to hammer out pecs and a six pack with some of my jewelers tools. Hm. Not so good, there. The metal kept crimping. Never one to be discouraged, I grabbed the pestle from my mortar and pestle set and used that instead. I folded up a fluffy towel and placed the copper atop it. Then I made circles in the metal. Don't press to hard, copper is very malleable. I pressed down and "drew" a little circle, then just kept going, rotating outward until I got a "muscle" the size I wanted. The sheet of copper will begin to curve as you work it, which is all to the good, because you want a bit of a curve to fit the body. It took about 20 minutes for the entire breast plate.
When it was completed, I had what looked like the front half of a tank top, with muscles. I recommend cutting out the basic tank-top shape (scissors work just fine) before you start making the muscles.
I glued some foam to the inside, for comfort. I spray painted it red, because I didn't have any magenta paint. Nobody noticed. Then I glued the whole thing to a shirt. I'm lazy that way. If I'd had more time, I would have used rivets and soft suede straps.
Then came the boots. Um, yeah. I tried to make boot leggings out of copper, so that I could get the texture and all; but I ran out of time. Halloween morning saw me frantically cutting leather leggings and spray painting them silver. I used the "pestle technique" to make the knee caps, stuffed 'em with foam and glued them to the leggings. They didn't hold up very well. If I make another set, I'll add a strip of copper to wrap around and hold the little buggers in place.

Last but not least, the face mask. You'll love this. I cut the leg off of a pair of sweatpants that my son had outgrown and had him pull it over his head until it was rather snug. Then I drew the cut outs for the face, ears and top. Snip, snip, and Voila! One awesome face mask. No sewing required.