Just going to try to piece this together a little more, speculation mixed in.
Sounds like Robyn was stringing Forester along. He was being used and is in denial. Robyn appears to have been "using" GG though obviously he had much more sinister motives at play. Robyn thought she was playing him but little did she know what he was capable of.
Something must have happened that Monday night, the night of "this sucks." He got angry at her over something. They probably both had been drinking. Something caused him to erupt. She then felt very uncomfortable and sought to reach out to a safe point, Forester. Perhaps they had argued while out partying in Aruba that night, maybe she flirted with another man, he got angry, she sends her "this sucks" message, they get back to the room, she refuses sex, and the storm within him starts to really brew. Then he catches her messaging Forester that she loves him, blows up with rage, and he then takes her for a drive and strangles her, disposes of body, reports her missing, etc.
The Holloways flew in cadaver dogs, maybe the Gardners should do the same. Justice for their daughter may depend on it.