At the end of Constantine, what did Lucifer mean when he said
looks like somebody doesn't have your back anymore
when Gabriel went to smite Lucifer?
He meant that God is not with Gabriel
Gabriel is trying to smite Lucifer with all his God-given might... just to be easily rebuked by Lucifer. So it is obvious from this, that God doesn't approve Gabriel's plan to bring hell on Earth.
Here is the script (italic parts spoken by Lucifer):
This world is mine. ln time.
You, best of all of us, Gabriel...
...should understand ambition.
Son of perdition.
l do miss the old names.
Time to go home, son.
l will smite thee...
...in his honor.
Looks like somebody doesn't have your back anymore.