On your DNA Tests page, under your AncestryDNA test, you will see something simliar to
"Test URL: http://www.wikitree.com/index.php?title=Special:DNATests&u=6035205&id=6
Added by Peter Roberts. Last edit on Mar 1, 2014.
[edit or enter more details] [delete test]"
Click on [edit or enter more details] and then add your GEDmatch ID.