Researching an upgrade to GRX Di2, am aware from this forum (as well as Shimano) that the GRX 2x crankset is pushed 2.5 mm outboard and thus requires a GRX FD. However, what about the opposite: using a GRX (Di2) front derailleur with an existing Ultegra 2x crankset that came stock on a 2020 Trek Checkpoint SL6?
Is there any particular reason you want to mix FC-R8000 cranks with a GRX Di2 FD-RX815-F front derailleur? the Ultegra Di2 FD-R8050 will get along just fine with GRX Di2 rear derailleur as Di2 sees it all as part of the road family.
the GRX front derailleur, including the Di2 FD, is shifted +2.5mm outboard to match the GRX cranks. I would not count on it working with your Ultegra crankset. that being said, depending on where the frame manufacturer's actual braze-ons lie relative to Shimano spec, the chainline issue may or may not work. unless you've bought smaller chainrings for your FC-R8000, however, your large chainring is too large for the FD-RX815-f, which is designed for a 48T max. if you have purchased the Ultegra 46-36 chainrings for cyclocross, you'll be even further from the 17T capacity of the GRX front derailleur, so I'd treat the front drivetrain as a package deal, and if you must buy one before the other, maybe buy the crankset first.
There was a post on the S-Tec (Shimano bike shop employee site) community forum where a user said that on his Di2 equipped Trek Boone, he was able to get the GRX crankset to work with his old (non-GRX) front derailleur. I wouldn't expect it to work as well in the opposite direction, but the location of the FD braze-on on the Checkpoint might be slightly different one way or the other.
According to a post from 9 months ago on the S-Tec forum, you could even use the RD-RX-817 with a front derailleur, albeit with some limitations, and no official word on "whether or not that will be 'fixed' in a later patch."