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I've had the same problem with my Gitzo. . It seems to happen quite a bit, actually, with all 3 of the foot inserts.
I've used various kinds of glue to put them back on with. . I know I've used Gorilla Glue and the construction adhesive that comes in a tube that you dispense with a caulk gun (just because that's what my brother had lying around when the foot came off at his place). . I've also used the glue that comes in a tiny squeeze tube, such as Crazy Glue, but I have no idea what brand it was ... whatever kind the Dollar Store sells.
Whichever glue I've used, eventually the inserts come off again and have to be re-glued. . Well, actually that isn't exactly correct. . Usually the feet come off out in the bush and I don't notice until it's too late to recover the foot. . The feet also often come off in the mucky bottom of a marsh, where the mud clings to everything and wants to pull the boots of my feet. . When that marsh bottom claims a foot there's no sense even trying to look for it. . Then when they come off in Pika rockslides they typically fall through the crevices between large boulders, and there's no way to recover them.
Sometimes I just have to go and shoot for a day or two with no foot on the bottom of the tripod leg. . Of course, this exposes the bottom of the leg to potential damage and breakage. . But the shoot must go on!
Salt water really wreaks havoc on those foot inserts. . It seems to erode any glue bond, so when I use my tripod in salt water, or even brackish water, I pretty much expect to lose the feet.
I gave up ordering replacement foot inserts from Gitzo a couple years ago. . It's too expensive for something that's just gonna fall off again. My brother is a contractor and hobby machinist, so he makes up things that work as replacement foot inserts. . When he's not around (which is often) I just get thin-walled black plastic pipe at Home Depot. . It fits very snugly over the lowest leg section. So I cut it long so that it protrudes an inch or three past the bottom of the tripod leg. . This keeps the exposed end of the carbon fiber tubing from coming into contact with the ground (which could eventually cause it to fray apart). . Of course, this prevents me from being able to retract the tripod legs all the way, so there is that caveat.
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"Your" and "you're" are different words with completely different meanings - please use the correct one.
"They're", "their", and "there" are different words with completely different meanings - please use the correct one.
"Fare" and "fair" are different words with completely different meanings - please use the correct one. The proper expression is "moot point", NOT "mute point".