Install this vehicles.meta in this location:
update\x64\dlcpacks\mpgunrunning\dlc.rpf\common\data\levels\gta5

This vehicles.meta will enable the HVY Nightshark, Jobuilt Hauler Custom and Jobuilt Phantom Custom to spawn with even more trailers, including Gunrunning DLC trailers.

If game keeps updating and restores vehicles.meta to original state, turn off internet or use GTA V Launcher Bypass by Unknown Modder