Well,
Whether you like brody's comments or not. He is right.
The additional fans in your iMac won't, necessarily compensate or work fast enough if the rest of the temp sensors in your iMac are not reporting accurate temperature information.
The CPU and GPU are, typically, the hottest running components in an iMac. WIth the GPU temp sensor not working properly, failing/failed or broken, your iMac will NOT be reading accurate enough temperatures to ramp up the internal fans enough to keep your iMac properly cooled resulting in a possible burn out of the GPU and the board it's mounted to, as well as, possible, potential damage to the iMac's logic board.
With this one seemingly small mistake, you have a potential of burning up your Mac's logic board to the point of failure.
You have, indeed, " screwed the pooch" on this one.
You're a perfect example of why I discourage many users NOT to try to tear into their expensive Apple iMacs and start a whole DIYing of their iMacs.
I consider myself a long time, long term, very seasoned Mac DIY upgrader for many tower, desktop style Macs, but have "ON PURPOSE" stayed away from doing any of my own hardware upgrades on iMac models because Apple purposely designs these with no user serviceability inside.
The reason for this is just, EXACTLY, what has happened to you.
Too many different sized fasteners and too many tiny, thin, fiddly and finnicky cable connections that have the potential to get damaged or broken, either at the teardown stage OR during the reassembly stage.
The only iMac model I ever felt comfortable enough with to actually do some hardware upgrades on were the older G4 desk lamp style iMacs which had easier access to the optical drive and hard drive than the new style, aluminum iMacs do now.
I will never do any teardown of these newer model iMacs.
I'll have an Apple Authorised reseller/repair center do any repairs or hardware upgrades to my two 2009 and 2010 iMacs.
Thank you very much.
Which brings me to my reply.
Apple Stores won't work on your iMac, but Apple Authorised resellers/service centers should be able to do the work on your iMac, instead, and fix the GPU sensor issue.
You will need to find such an Apple Authorised repair center in your general location to bring your iMac to to get the temperature sensor replaced.
Good Luck to you!
😉