iFixit is not correct.
You absolutely do not need a heat gun under any circumstances. iFixit is a useful resource, but they've had a massive stick up their ass over Apple's recent design decisions (especially regarding using adhesives), and on this point they are 100% wrong.
All you need to open a 2012 iMac is a thin plastic tool - either a guitar pick, or a pizza cutter. Or, if you go on ebay and want to spend the cash, you can get the official tool that Apple sends to their authorised repair shops - a small plastic cutting wheel.
At no point anywhere in Apple's service manual for the iMac does it mention the need for a heat gun.
Really!? I had no idea, I thought that iFixit was the best one to trust. So then you can just cut through the adhesive, and other than that it's probably similar to the old one