Imagine how hurtful it must have been for Crowley to see Gabriel, the person who literally took an active part in the process of traumatising the man he loves into the mindset that he cannot be happy and free, get a happy ending. To see the man, who was one of the reasons Aziraphale haven't even acknowledged their relationship for such a long time, because they were on "opposite sides". The man who was one of the reasons they couldn't be together, even as friends, for 6000 years. The man who wanted to execute Aziraphale with a smile on his face and "shut your stupid mouth and die already" as his "goodbye". Seeing this man getting what Crowley has always dreamt of: being able to run away with the person he loves, leave Heaven and Hell behind and just be together with his beloved - in Soho or on Alpha Centauri, literally wherever, because "Heaven is wherever Beelzebub is". Crowley seeing Gabriel get it so... so effortlessly, while he has worked for it so long, nearly died because of it and still haven't been able to get that, to get the happy ending and sever all the connections with Heaven and Hell. It must have hurt like hell (most literally).
