So a Gate connection to the Japan Self-Defense Force from the anime Gate will connect to the universe of the last piece of one of the following media you watched (TV, Movie, Video Game)
So for example last game I played was RE5 so nothing to bad unless Weaker releases the virus on the other side.
Last show I saw was Invincible so the Viltrumites and Super Villains clear house.
Last movie I saw was some Werewolf movie so the Japan Self-Defense Force stomp kinda.
Oh it's television show, movie and game... Errr
Then it that case...
Loki for the television show.
Shin Godzilla for the movie.
Total War Warhammer III for Video Game.
By
Loki it's actually the whole TVA/Multiverse and timeline nonsense. Maybe not much will change, especially since the whole point of the
Loki show is the multiverse thing going on... so this world would likely be its own timeline. Maybe weaker then average due to lack of superhumans but really not a big deal IMHO I feel.
Shin Godzilla as forementioned would be metal asf since it'd basically be two Japans I suppose, one Japan can support the Shin Godzilla one in its time of need, especially with the tens of billions in destruction and recovery that will be needed. But outside of humanitarian concerns I suppose the two worlds aren't too dissimilar to be honest.
Total War Warhammer III takes place in the Warhammer Fantasy world so that would be metal asf. The Warhammer Fantasy world in TWW3 is leagues more formidable then the Gate Fantasy world AFAIK and approaching it holistically would likely provide a fair bit more impediment to conquest, but I feel there isn't actually much that can reliably conquer or defeat a modern combined army. Plus the lack of Winds of Magic in the real world would blunt the Warhammer fantasy races from making a proper initial impression on Japan itself unfortunately. Still the counter-campaign to conquer the Warhammer Fantasy Realm, I suppose depending on where the gate is, would be highly problematic.
My last campaign was with High Queen Khalida, a Tomb King faction so if the Gate opens in her territory, she might actually be pretty amicable to the strange Humans as opposed to attacking them outright. And she'd probably love modern artillery, since she grants her army loads of ranged attack benefits already so they'd be even more invincible. In return, the Mortuary Cult can mummify and bring Japanese dead back from life. Bonus points if they do so without being asked first. 💀
ONWARD TO THE YASUKUNI SHRINE! We're bringing back Grandpa!