Why is character David Gabriel not on major crimes?

In an ongoing series, some characters may be 'written out' for various reasons, including an actor's unavailability, the end of the character's arc, or other.

You answer may lie in this plot point, quoted from Wikipedia, below:

"At the end of Season 4, Gabriel has broken up with Daniels, and apparently they did not part on the best of terms. Their squabbling becomes enough of a distraction for the squad that Daniels has to be transferred to another department."