Project Treble has been hit or miss for the users running GSIs in the development community. It has helped custom ROM developers get their actual custom ROM to devices faster, but it's not always the best experience for those who have chosen to just use them on their device. A great example of this is the Pixel Experience GSI which had impressive results on the Galaxy Note 9 by itself. However, there were some things that didn't work quite right (such as registering a fingerprint) but we see these quibbles in full custom ROMs as well.
Samsung Galaxy Note 9 XDA Forum
Still, a custom ROM is generally preferred to a GSI, when it's available. XDA Senior Member Rainbow_Dash, with significant contributions from members Ivan_Meler and jesec, just published a port of the Pixel Experience custom ROM over to the Exynos version of the Galaxy Note 9. This isn't to say that the custom ROM is flawless at this stage though, as the developer adds the following notes:
- Sometimes after setting the brightness to maximum, it doesn't want to lower its value and gets stuck. is fixed only by a reboot.
- HWC is disabled.
- The signal bar always shows 0, altho calling and data have no issue and should work as intended.
You will also want to make sure you are on the CSA2 based vendor and firmware but there isn't any need to flash Gapps because they are already included.
Check out the Pixel Experience ROM in our Galaxy Note 9 forum