So I just took notice something from my theory posted earlier (surprised I didn't get a whole bunch of replies as I original had anticipated.) So I am going to do a short short version.
Now, if any of you out there has seen based horror films, studied lore on creatures and know how certain fiction and fantasy magic based elements work as well as the typical CV lore: then you will have no issue understanding this. If not I suggest doing some serious heavy reading, making a brush up with old horror monster films...etc.
First off -
His wife was murdered, by the manipulation of Zobek. Now if you pay strict attention that the only way for Zobek to do this is manipulate Gabriel while he is asleep - now, by taking note that he is fact Death. Hence why at the end he calls him "Old Friend." in the original Lore Dracula and Death are friends. Now if you take into consideration how does a mortal make a pact with Death? Well... If you look they shook hands after the Ice Titan Battle, and then later after destroying the Black Knight Golem (if you read the monster's bio) that it required the 'corpse' of a murderer. Well... In order to seal this body to its armor Claudia's father had to use a form of Necromancy in order to give the corpse life and the Armor its bonding seal (i.e the glove) in order to bring it to life. Which in theory: the being needed souls to survive. What exactly does Death need? :)
So by in context: the Gauntlet (Due to the pact) is forced upon Gabriel due to the dark magic of Death which then gives Zobek the power of dominion (another old CV reference; which isn't spoken in game but its obviously seen.) over Gabriel. Now if you pay strict attention to the Dark Lords there is a lot of portion in the sequences in which Gabriel uses his gauntlet to hurt them. So at this point they had now been touched and claimed by "death", which is how he was able to "reclaim" the other two powers of the Dark Lords (you had obtained the first power freely due to the pact.)
If you also pay strict more attention he is bitten (QTE movie sequence damage is taken) by three vampires and the werewolf. Due to this nature it allows him in an actual turning in lore: A vampire must bite their victim and then then must let them feed on their blood in order to become turned. In which is exactly what Laura did.
But basically prior to the events - in the end as to why he went further and further into darkness you will want to keep in mind that Zobek, Cornell and Carmilla and of course Pan had been trying to tell him to the truth, but in the end he had been lied to (as the three dark lords) by their heavenly father. Now what drives him further into madness is the fact that he had been lied to about the God Mask and lied to by his very own wife.
You will also take notice he will have the Devil's Mask at first cut scene (which if you pay attention to the main hiding ending, you can see the mask has actually fused with his skin.) You will then want to take in to consideration at the very ending that Gabriel 'absorbs' the power from TFO at the end with the touch of the "Gauntlet". This is basically a reference to the "Power of Dominance" found in Aria/Dawn story lines. Basically, its more than like I am assuming as of now he is not a vampire yet - he just killed TFO thus he has acquired the title: The Prince of Darkness. Reasons to this is at the TFO in the monster bio was known as the: "sovereign" of the elemental Plane "of Darkness"
Sovereign ___ Hence "Prince of Darkness"
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–noun V
1.
a monarch; a king, queen, or other supreme ruler.
2.
a person who has sovereign power or authority.
3.
a group or body of persons or a state having sovereign authority.
The basic outline from what I am seeing from this is personally Gabriel returns to the Castle, and since obviously it was known as "Chaos entity", he will need to establish his "Power of Dominance" over the "Demon Castle" - before he becomes a Vampire. Upon his 100% turning and the Castle is now "his", it will grant him his 100 year regeneration Cycle (Satan comes back 1,000 years.) so who is the real threat here? He needs to establish a foot hold and create a vast army from his "Demon Castle" as you can see Medusa, Gaibon (or is it one of them ston-lisk thingies?) and Slogra are all in the tower at the end. In which he will either overthrow Satan or declare dominance over him and then will challenge God and Humanity at the very end just as he is suppose to do.
So in a sense: the title Resurrection is basically Gabriel's "resurrection" as the Prince of Darkness but is not quite yet a vampire.