How do I replace the display gasket on a 12" MacBook?

You’ll need to first take the display off following this guide Retina MacBook 2017ディスプレイアセンブリの交換 then you’ll be able to grab the end just behind the hinges (either side) then carefully pull. The rubber has a bit of a rib which is grabbed by the edge of the glass and metal frame. Putting in the replacement is a bit tricky! As you need to start from the middle and make sure you don’t over stretch the rubber. I find a little bit of WD40 sprayed on a paper towel which you rub on the rubber to just barely coat it makes the job easier. It only takes a drop or two! So go easy as you then will have streaks on the display if you go overboard. Once in, I use a cotton pad to wipe the rubber down with isopropyl alcohol (again damp not wet) often using multiple pads to clean the WD40 off. Then a micro cloth with a glass cleaner wiping way from the glass.