I believe the original saying was "Beware of Greeks bearing gifts", based on the Trojan horse story. In more politically correct times we replace Greeks with "strangers or enemies".
The gist fo the saying being that one should be suspicious of a gift give by someone who has no reason to give it
I think the other saying that you are mixing the greeks one up with is 'never one to look a gift horse in the mouth' which describes someone who will take anything that seems beneficial without considering what might be the more serious consequences behind it - if the trojans had looked in the mouth of the gift horse they would have seen the greek warriors hiding inside