Can they just ship devices with TWRP
And a link to OpenGapps.org or something and a flashy guide to tell the users how to flash the Gapps without drooling?
(Legally speaking, all is fine. A company whose name escapes me has done this with Hackintoshes. They ship motherboards that run "any OS you want" with a faint whisper in the background: even macOS.)
Edit 2: There's the name, projectQ, and it's not as "silent" about macOS as I said: https://www.engineering.com/3DPrinting/3DPrintingArticles/ArticleID/5410/A-Motherboard-that-Runs-any-Operating-System.aspx