Pretty much mentions the GPU in the first lines with references to Nvidia further down.
Try starting up in Safe Mode,
How to use safe mode on your Mac – Apple Support
This will run disk repairs and clear caches, it takes a long time
to start up in Safe Mode so be patient.
Restart normally.
Then try this, Reset NVRAM or PRAM on your Mac – Apple Support
Click on the top left of your screen select About this Mac and take
a note of the graphics card details, go to the Nvidia website and see if there are
any drivers available for the OS and mac you are running, install if any
see if it makes a difference.
You could try reinstalling High Sierra again, you do not need to erase your mac first just install it over what you have.
Get the High Sierra installer here,
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Go to Download OS and click on macOS High Sierra
this will redirect you to the App Store High Sierra page it should say Get or Download.
Back your mac up before you reinstall the OS.
If these don't work and you do not get any further forward with anything I have mentioned
or any other contributions from other it may be time for a new mac as the graphics cards in MBPs
cannot be replaced.