Seven possibilities
1 separation happened before homicide (suicide)
2 separation happened before homicide (3rd party foul play)
3 separation happened after homicide (suicide)
4 separation happened after homicide (3rd party foul play)
5 Brian killed gabby (self defense)
6 Brian killed gabby (manslaughter)
7 Brian planned to kill, then killed gabby (murder)
Uk daily mail reporting that Brian was possibly sighted at baker, Florida panhandle
Brian might be attempting to get to Mexico border on foot(?)
To be complete there is another possibility
8 Brian conspired with a 3rd party to murder gabby
However the Utah police incident appears to indicate that Brian was low on funds. That might just have been a story Brian made up for the police, but clearly they are not staying at Hilton hotels along their way, so personally I discount the 3rd party hit person possibility.
COVID!
“… 7 Brian planned to kill, then killed gabby (murder)…”
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You forgot number 8, Gabby told the dirtbag she was leaving him (or at least she no longer wanted a boyfriend/girlfriend relationship…just “traveling companion”) and once he has an opportunity to have her in a remote/isolated carried out the murder. IMHO, that’s the most plausible situation given the overall situation.
One thing I heard today was that the female park police member at the traffic stop location had a long talk with Gabby counseling possibly ending the relationship at some point at a safe location.