What's about ?
It is the juxtaposition that is funny to some readers.
"gumboots" is a very American phrase - the English use "wellies" or "wellingtons" to describe the same footwear.
So to have some one from France using a very colloquial brand name for a type of shoe as their internet handle seems amusing.
On the forum there is a thread about why people have chosen their name/handle and/or avatar.
One could ask Trip why he is such a fan of Boston Legal :-)
Or what is an elephant doing in Australia :D