First theirs the obvious appearence, both the Gatekeeper and Vadderung have one eye of what appears to be steal, and according to most myths about Odin, he sacrificed his eye for knowledge of magic.
Bear with me. Vadderung clearly knows about the Island and it's purpose, he outright states so himself.
"What I mean" he said "is that I know about your island. I know where it came from. I know what it does. I know what's beneath it."
Then Vadderung shows his extensive knowledge of the island and time in relation to magic, which seems perfectly in line with knowledge a Gatekeeper has.
"Idiots" he breathed. "Even is they could defeat the banefire..."
"Wait" I said "Banefire"
"The fail-safe" Vadderung said. "The fire the island showed you"
....
"It is the only way" Vadderung said. "If anyone managed to set free the things in the Well..."
....
"Not bad" Vadderung said "The end"
....
"The island cannot accept it as a possibility" Vadderung said absently.
All of this suggests a tremendous amount of knowledge by Vadderung regarding the Island.Next Vadderung goes into a detailed discussion in regards to time, space and magic.
Vadderung waved a hand. "Nonesense. There are laws that govern the progression of time in relation to space, like everything else."
"Meaning what?"
"Meaning that the echoes caused by temporal event are proportionately greater than the span of time that was bridged" he said "Had the attack been launched from a century ago or hence, the echoes of it would have begun far, far in advance of the event--- centuries ago. These echoes have appeared only within the past few days. I would guess roughly that the attack must originate only hours from the actual, real time occurance"
Now first their is a description of what limits the Gatekeepers, Foresight/Hindsight in Proven Guilty thats relevant.
"Because if he significantly altered what happened with his knowledge of the future it could cause all sorts of temporal instabilities. It could cause new parallel realities to split off from the point of alterations he couldn't predict or kind of backlash into his consciousness and drive him insane"
....
"But if your car never got stolen" Bob said "then how did he come to know to come back and warn you?"
I frowned
"That's paradox and it can have all kinds of nasty backlash. Theory hold that it could even destroy our reality if it happened in a weak enough spot."
.....
"If it's done subtly enough, indirectly enough you can get all kinds of things changed. Like for example, he tells you that your car is going to be stolen. So you move it to a parking garage, where instead of getting stolen by the junkie who was going to shoot you and tale the car on the street, you get jacked by a professional who takes the car without hurting you- because by slightly altering the fate of the car, he indirectly alters yours"
Then we get the explanation that counters Bobs explanation of the problem of Paradox.
"Paradox is an overrated threat. There is... a quality similar to inertia at work, Once an event has occured, there is an extremely strong tendency for that event to occur. The larger, more significant or more energetic the event, the more it tends to remain as it originally happened, despite any interference."
I frowned. "There's... a law of the conservation of history?"
Vadderung grinned. "I've never heard it phrased quite like that, but it's accurate enough. In any event overcoming that inertia require's tremendous energy, will and a measure of simple luck. If one wishes to alter the course of history, it's a far simpler matter to attempt to shape the future."
.....
" A twinned universe" Vadderung said "A new parallel reality, identical except for that event. One in which you never existed and one in which you failed to kill your grandfather"
.....
"Meddling with time is an irrationally, outrageously, catastrophically, dangerous and costly business. I encourage you to avoid it at all costs."
.....
"Because the energy requirements are astronomical" he said "Bridging a temporal gap of any length is something utterly beyond the reach of any mortal practitioner alone. Doing such a thing and then trying to project the spell over a distance as well? The difficulty of it would be prohibitive. And do not forget how much water surrounds the island, which will tend to mitigate any energy sent toward it--- that's one reason the Well was built there."
Plus a little subtle hint at foresight/hindsight
"Perhaps" he murmured "you already have"
Theirs also myths about Odin, hanging himself for more knowledge or power. which reflects Vadderungs statement to Harry about death.
"Fewer try it then you might think"
....
"Nothing that significant just happens, Dresden"
.....(later on in the chapter)
"Wizard...you have been dead and returned. It has marked you. It has opened doors and paths that you do not yet know exist, and attracted the attention of beings who formerly would never have taken note of your insignificance"
"Meaning what?" I asked
There was no humour at all in his face. "Meaning that now more than ever, you are a fulcrum. Meaning that your life is about to become very, very interesting"
Okay so with all this knowledge about Old Ones, and Outsiders and the Island, plus the similarities in appearence, and mystique of knowledge theirs also another fact that contributes to the theory that Vadderung used to be the Gatekeeper.
We know from his reveal that Vadderung is Kringle.
"Many, many mantles are worn--or discarded-- on Halloween night, wizard."
...
I saw Vadderung's wolfish features lurking inside Kringle's
So we know mantles can be discarded.
Finally the Outer Gates need an army to combat the Outsiders, before it was Winter it would make sense that it would be Einherjaar, Warriors dying in battle over thousands of years, would create a large army to which battle could be joined against the Outsiders at the Gates.