@Overlord24That's unrealistic to say the least. If we treat fandoms like nations, they would not simply just give up technolgies. You don't see Russia or America selling nukes and nuclear plans willy nilly now do you? That would also change the way certain dynamics would change. Giving guns to the HP universe would change everything since now you no longer have a focus on pure-magic, but instead guns and maybe gun-magic.
Lets take the entire Percy Jackson universe and pit it against a single faction from 40k: the Imperium of Man.
While yes PJ has magic and stuff on his side, they don't have the numbers. At best they have less thank 100,000 people worldwild and even if that number is off, it doesn't matter since the main military arm of the Imperium is in the trillions and they see the loss of billions of troops over a small hill to be a success. This isn't going into the Space Marines (most of which would just laugh at what the greek demigods could do), the Adeptus Soritas, the Adeptus Mechanicus or even the Imperial Navy. The Imperium can and does nuke entire planets and even goes so far as to destroy all living things on it via Terminatus. PJ would loose purely from attrition, demi-gods are still human at the end of the day and there's a limit to how many people they can hold off.
Sure you could say that the Gods would intervine on the side of the demi-gods but that would be canon lore if they have to consistently do it. And even then there's no chance that they'd win out and it'd just be a battle between the greek gods and the Imperium of Man, of which the Imperium would just simply fuck off, Terminatus the entire planet and routinely come back to purge the planet every so often.
Of course, if we're following the way "magic" is dealt with in 40k, the Gods would have very little power given the way the Warp works in that the more people believe in something, the greater the chance it will happen (simplified version but gets the point across), Jesus Christ would have more power than the greek gods and that's assuming that they aren't just eaten by the Chaos gods.
All of this and that's no considering the Eldar, the Orks, Daemons, Chaos, Tyranids; if all of 40k was to be brought down upon the PJ universe/fandom, there is no way for PJ to survive.
As for trading tech and stuff, discounting the fact that that is literally not how it works, the Imperium would see PJs has heretic and refuse to do any dealings with them that do not involve converting or purging.