Okay here I go. First Tony's suit contains nanites that cant replicate any kind of technology, so they can replicate the Gauntlet, furthermore Tony already made the gauntlet that the Hulk used, so it stands to chance that his suit can replicate the technology. Lastly the avengers had the original Infinity Gauntlet that they got from OG Thanos, so either Professor Hulk, or Tony could have studied their design in that 5 year time skip, they did not just do nothing between those years, well except Thor

Thanos: Tony Stark was able to build this on a dirt planet! With a box a scraps!
Eitri: Well I'm sorry… I'm not Tony Stark.