Looking for a medieval fantasy system that accomodates high tech relics and ruins, kinda like xenogears?

Hello, I want to run my own setting that's basically a fantasy world that sprouted after an apocalypse destroyed an advanced civilization on the surface thousands of years ago.

The idea is that a standard adventuring party will gain access to more and more advanced technology as they progress through the campaign, swords are replaced with energy blades, bows with guns and/or automatic ballistas while magic users get tools to help them be more efficient (like a book replaced with holodeck or something of that sort).

I'm planning to do this in GURPS but figured there could be an easier way by just finding a game that does exactly that sort of thing. Class based, skill based or whatever systems are all good, I don't mind learning anything new. Also please no DnD.