I ordered a GT4 lip through MK-Rennsporttechnik a while back and finally just had an evening with some free time to install it on my spare bumper, I also put the canards on. The fitment is great (hard to tell in the photos).
I tried to mimic the gasket that my OE GTS lip had. I couldn't do it as well as BMW obviously but this seemed to do the trick. I ordered a rubber strip through Amazon and applied it so it's sandwiched between the bumper and lip, this also fixes any minor fitment issues as it fills in the gaps.
Applied 3M double sided adhesive (towards the back of the lip so it isn't visible when installed) so the rubber strip wouldn't walk around as I was laying it on the lip
Lay the weather strip down and cut width
Trip the inside edge of the weather strip
All done
I did have to trim my spare bumper for the OE GTS brackets since I left them installed on the car. I was afraid to remove them since they're all riveted in place, and the brackets themselves are super hard to source. I'll admit though, I am worried that the brackets are more prone to flexing/warping the left or right side now that there's no bottom support linking the two together. I'm also concerned that if I bottom the lip out the chassis mount will shatter the inside edge of the carbon brake ducting since it's super close on the inside edge. Either way the fitment on the lip is excellent and I'm very happy to have held out for a decent GT4 lip as opposed to the cheaper knock-off ones like I had on my previous E92.