I think taking the "Gabriel ascended into Heaven" theory is what the developer's want us to believe happened, but I don't think this is the case. Too obvious.
Recall that in the game the the original founders only learned of the ascension technique through years of study and research. Gabriel did not have this knowledge. I also find it hard to believe that he would pursue this knowledge after defeating Satan knowing full well what the consequences would be - leaving his corrupt/darker half behind.
Playing through the game again, and I'm noticing that everyone keeps mentioning Gabriel's "destiny". At one point Pan (in eagle form) even says to Gabriel that he's "not helping him". Later on, Pan also says that Gabriel "cannot escape his fate". Now, go back to the prophecy "It is written that a pure hearted warrior will claim the power of the Lords of Shadow as his own". Sounds pretty much what was prophesied is what happen. A pure hearted warrior (Gabriel), claimed the power for his own. Now, his reasons for doing so were mainly an attempt to bring back his beloved, but even so, the prophecy doesn't speak of motivations, only what would happen.
In the Titan Graveyard a note from one of the fallen Brotherhood Knights alluded to as well, basically saying something like anyone who would be powerful enough to actually take down the Lords of Shadow, would more likely than not become a tyrant themselves because of how powerful they would have to be to do it in the first place.
Now, as for Gabriel not having the knowledge the founders of the Brotherhood of Light had to make themselves as they did, that is moot. The reason being that before you even defeat Satan, Gabriel's soul has already ascended to Heaven. Why else would Satan have said to him "So, He has abandoned you too?". Just moments before, Gabriel was dead (physically speaking) because Zobek killed him. Being brought back to life hardly seems like being abandoned, unless you consider that perhaps Satan was referring to something else. What had happened to the BoL founders is what happened to Gabriel. His soul went to Heaven, but his body stayed on Earth, and just like with the LoS, his body became cursed. THAT was his fate, that was his destiny. He claimed the power of the Lords of Shadow as his own, became extremely powerful, but also cursed.
One more thing that popped into my mind. In the original CV folklore, the Belmonts had super-human power (see CV3 and Legends). Perhaps Gabriel is half angel or something, and since his spirit is in Heaven, his body eventually transforms into something demonic. Also of note, within the various notes on the monsters, it is mentioned that vampirism need not come from an infection so to speak, but can also be contracted through a spell or a curse.
Lastly, I agree with your assessment that Gabriel/Dracula does not "seem" evil per-say, but tired and beaten down. It's possible that even though he attained to power of the LoS, since he did it for good reasons, and didn't actually want the power, when his soul became separated from his body, he didn't become evil at heart, only in body. So then, why was this his fate? One of the reasons given is so that he can atone for his sins. But I think another is so that he can defeat Satan once again. Perhaps this second and final defeat is how Gabriel will atone?