in anime's first season one gear(twilight healing) changed user by killing the user and ritually transferring the gear to new host but if you resurrect the real owner they get their gear back
you can crate artificial gears by sealing strong monsters or some other procedure ( their local mad scientist angel creates for whole secondary team) but there is no i invest stuff to my gear so i can use more powerful form you need to grind levels to unlock full power and on issei's and vali's case they need to quest/ sub plot to achive full power
as your incarnum coment vecna just one character uses incarnum style gears( he has same gear but takes multiple slots/ abilities