Keane had scored twice in the first-half before former Liverpool captain Gerrard was introduced during the interval to the 27,000 crowd at the StubHub Center in LA.
Gerrard, eligible to make his debut from July 18, told supporters: “I can’t wait to get my boots on and play in front of you guys.
“I’ve been in Liverpool all my life. But this is a good place for me to come and enjoy my football, and hopefully add a trophy to my collection.”
Keane, not to be upstaged, completed his hat-trick on the hour before Sebastian Lletget added a late fourth.
The Republic of Ireland captain’s fellow international striker Kevin Doyle opened his MLS account for Colorado Rapids in a 2-1 win over the Vancouver Whitecaps in the league’s American Independence Day action.
Elsewhere in MLS action, former Brazil star Kaka scored and was then sent off as Orlando City drew 1-1 with Real Salt Lake.
The former AC Milan star hit the net after just five minutes before Jaime equalised for RSL and the Brazilian failed to finish the first half after he launched into an uncharacteristic lunge at Javier Morales and was dismissed just before half-time.
David Villa scored twice as New York City FC won 2-1 against the Montreal Impact.