/-------------------------------------------------------------/
Add-on Installation :Put ww1pack folder in Mods/update/x64/dlcpacks
/-------------------------------------------------------------/
Open OpenIV And navigate to Mods/update/update.rpf/common/data
And extract dlclist.xml to desktop or anywhere you want.
Open dlclist.xml and on top or at the bottom add this line
<Item>dlcpacks:\ww1pack\</Item>
/------------------------------------------------------------/
Save the file and put it back into the update.rpf/common/data

Spawning vehicles : use Enhanced Native Trainer or Simple Trainer to spawn them.
Fokker Dr. 1 / fokkerdr1
Sopwith Camel / sopwithcamel
/----------------------------------------------------------------/
If your game crashes on startup download this and put it in the same folder like you did put dlclist.xml (mods/update/update.rpf/common/data)

Link -  https://www.gta5-mods.com/misc/gta-5-gameconfig-300-cars