Pretty sure we're talking about the same thing.

FiveM is a variable I can't account for as I've never used it, but it looks like what's happening is that either the vanilla HD textures for those billboards are being loaded instead of the LS 123 Gang ones or the LS 123 Gang HD model, if one exists, isn't being loaded.

The original HD vanilla textures for those billboards in a normal SP modded game are loaded from 'dt1_22_bld1x+hi.ytd' & 'dt1_22_bldg2_detail2b+hidr.ytd'. Unless of course, the entire billboard model ('dt1_22_bldg1.ydr') has been replaced, in which case the texture & '.ydr' location could be different & you could then use CodeWalker to track down their location/s, if you don't know the name of the '.ydr/.ytd' that's used already (ie from the dlc etc).

If FiveM loads textures/models differently to a normal SP modded game you should be able to figure out the rest, just adapt the approach to suit your specific (FiveM) needs. In principle, it's pretty simple, the textures/model are being loaded from somewhere, find that location, whether model or textures & replace them (Hint: perform a 'Tools' > 'Binary Search...' in Codewalker for the name of the model/textures to find all their locations & then change them one by one (if multiple locations) to figure out which ones are being loaded by the game. Alternatively, load up CodeWalker while activating mods & the LS 123 Gang dlc ('View' tab for both) & move towards that location to see what's happening). Job done.

If you link to the LS 123 Gang model/textures/dlc etc download I can probably help further if required.