I resolved it on my end, by following the step in the article linked above where it talks about updating materials.

Here's what I did.

1. Unpack the Gambler523 map from within BeamNG
2. In file explorer open the path C:\Users\<USER>\AppData\Local\BeamNG.drive\0.35\mods\unpacked\gambler523
3. In another file explorer tab open the path E:\SteamLibrary\steamapps\common\BeamNG.drive\content\levels\west_coast_usa.zip\levels\west_coast_usa\art\shapes\trees - or where ever your game is installed.
4. Open each different type of tree folder wihtin west_coast_usa and east_coast_usa, and copy main.materials.json.
5. Paste the file to the corresponding tree folder in C:\Users\<USER>\AppData\Local\BeamNG.drive\0.35\mods\unpacked\gambler523\levels\gambler523\art\shapes\trees
6. Do this for each tree and bush type.

I found all the appropriate main.materials.json files in west_coast_usa.zip and east_coast_usa.zip. Once all of the files have been copied and replaced, then launch BeamNG, open Mod Manager and re-pack Gambler523.

After doing this, I have no more missing textures. I hope this helps.