If you have ever studied mythology, this should be familiar to you. This is just a recap. The Trojan War was between Greeks and Trojans. The Greeks gave the Trojans a big, wooden horse that was hollow inside and could hold 30 armed men inside the body of the horse. The Trojans then pulled the horse inside the city. When night fell, the Greeks came out and attacked the unsuspecting and drunk Trojans.
This is what "Beware of Greeks bearing gifts" means.
The phrase is from the Aeneid , Laocoon is trying to warn his fellow Trojans it may be a trap and in so many words warns against trusting the Greeks (in the ancient world the Greeks were notoriously sneaky)
But all accepted references aside, "beware of Greek bearing gifts" may have actually come from the Bible. This reference may have originally come in the gospels of Mark, Luke referencing the "antichrist"