Status: Enemies (Possibly Former after Gabriel sided with Michael at the end of Season 2)/Former Allies; Their feud has been alive for decades, They both hate each other. Gabriel is responsible for Alex's orphan status. Gabriel wanted to utilize him. Alex totally hates/mistrusts him but went to him at the end of Season One anyway.
This is the relationship between Gabriel and Alex Lannon. Gabriel was set out to end Alex's life since he was born. Decades of war was caused because of Gabriel's hunt for Alex and to Exterminate Humanity. Gabriel now wants to utilize/mentor Alex after years of one of the worst wars in human history. Alex went to him after season 1 was over, possibly to seek mentorship from Gabriel after Michael became overaged. During season 2, they had a strong disagreement before Claire's forces from Vega destroyed the hideout they were in. Gabriel later finds Alex in New Delphi and tries to grab him again. However, Gabriel's attempts to grab Alex result in failure.
Later during the second season, Gabriel's disbelief in Alex being the Chosen One is magnified into anger for him and his twin brother, Michael. Due to Gabriel being infected with the Amphora's darkness, Gabriel's hatred for the Chosen One is what influences him to release the Amphora onto Vega, killing hundreds of civilians inside the city. Gabriel also plans to kill Michael due to his bond with Alex.
After Gabriel is cured from darkness by his brother, Lucifer, he sides with Michael when he learns that Lucifer is looking to use Alex for his process in coming back to life. After a decades-long war, Gabriel finally sides with Michael and Alex and flys back to Vega when Gabriel and Michael figure out that Noma Banks betrayed Alex by siding with Lucifer.
Being this series was cancelled, one does not know what happened to the characters after the events of Season 2. After Michael and Gabriel rescue Alex though, it can be assumed that Michael made Gabriel and Alex forge an uneasy, beginning alliance. Due to Gabriel's war, making Alex an orphan, and killing the ones he loves, Alex stubbornness may get in the way of forgiving Gabriel for awhile. However, it can be assumed they maintained an alliance for Michael's sake and for the sake of Earth and humanity. Gabriel and Alex may have formed an unlikely friendship while defeating followers that protected Lucifer and eventually, they may have defeated Lucifer themselves. In the end, their differences would be put behind them and Alex could forgive Gabriel for his war on the Chosen One and humanity.