So I mentioned this to Jerry, but one of the major win-conditions for Morgott is currently missing from his profile, so I'm unsure how to due this matchup. Morgott along with the other demigods have type 1 and 5 immortality due to the removal of the rune of death in the Elden Ring. Thus, I don't see any way Gabriel could kill him normally. Morgott also possibly has type 2 immortality based on him having his cursed blood drawn out of his body and into who sword, as well as the fact he can survive until near the end of the game even after he is defeated.
Going off what is listed in the profiles however, things get a bit trickier. The two are fairly evenly matched, with neither having any prominent stat or ability that could beat the other. The only things I see for Gabriel is his higher LS and he can teleport. For now, I'll abstain until someone can maybe give a better argument for either character.