Stolen bikes turn up everywhere, Gumtree is a good place as anywhere for getting a legit bike.
Unless the ad is highly suspicious I would presume it is all above board, although I am sure stolen bikes have popped up here and there (Just like Loot/ebay/Singletrackworld/etc) - 99.99% of what you see on Gumtree will be fine.
Stolen bikes turn up everywhere, Gumtree is a good place as anywhere for getting a legit bike.
Unless the ad is highly suspicious I would presume it is all above board, although I am sure stolen bikes have popped up here and there (Just like Loot/ebay/Singletrackworld/etc) - 99.99% of what you see on Gumtree will be fine.