I'm adapting X-COM to GURPS 4e (I played a whole campaign from beginning till the end in Cydonia, using 3e).
Here is the equipment, the text is in Portuguese and the weight is in kilos, but it's mostly a selection of Weapons found in Characters:
http://www.gurpsnation.com/artigos/94-x-com-equipamento
The Aliens are here, in english. There still are some details to finish: how to make the melee attacks of the Chrysalid, Celatid and Silacoid dangerous to men with DR 50 sealed armor and still believable?
https://sites.google.com/site/thackbarth/gurps-x-com
I tried to remain true to game stats. The difficulty of downing a alien with each different weapon should remain the same as in the PC game.