Macbook Pro won't boot up
Have you looked into LangLangC's suggestion in this post: GPU problem - Boot Hangs on Grey Screen. Apparently, this fix works for NVDIA as well as Radeon chips. If you have another Mac, you can use target mode to access the failed MacBook; this post has some advice on that: Single-User-Recovery-Mode on High Sierra 10.13.6.
Also, this post might help you: https://www.justinsilver.com/technology/os-x-el-capitan-10-11-1-hanging-on-boot-fixed/#os_x_fix. 
Good luck!