I'm sure you can think of your own amusing comments for this present. I'm pretty sure they are available in the London Transport Museum Shop.
However, if you want to buy an unofficial Mind the Gap thong, click here to get one online.
Now whether these thongs are unisex is a matter of opinion. Imagaining a man in them does conjure up the image from the excellent Bob Rivers Song "Waking Round in Women's Underwear" and sung to the "Walking in a Winter Wonderland" tune. Listen to a snippet here (you need RealPlayer for this).