They have an article from CT JTAS that would be the nearest mechanically. Based on that, I would give them Weak STR (-4) and END (-4) (both with minimums of 1, obviously), but not saddle them with Amphibious (under Aquatic in the MRB) as they do not fit that key word. They also have career entry DMs (penalties for hands-on military, bonuses for Scouts, Citizens, and Merchants) and a penalty to use equipment meant for Human hands. They are given an odd bonus to Admin skill which doesn't have a precedent in MGT (the first rank they get is doubled).
if you have the GT write up, you already know that Newts are common across the Imperium.