Use OpenIV (with a mods folder) in "Edit Mode" then install to Grand Theft Auto V \ mods \ update \ update.rpf \ x64 \ data \ cdimages \ scaleform_generic.rpf \ and replace your current "hud.ytd" with the one from this folder

The changes made in this hud.ytd will add a HQ Glock Icon for the in-game weapon wheel UI, everything else within the file is entirely vanilla GTA V and unchanged.
Sadly, it is still impossible to override files from scaleform_generic.rpf via standalone add-on dlcpacks, so for now this is the only real way for it to work. 

If you already have a modified hud.ytd file used for other mods however, then simply use OpenIVs texture browser by double clicking on this file and opening it via OpenIV, search until you find the texture "weapons_all", export it somewhere on your PC, and then import it into the hud.ytd you were currently already using.

IF YOU DON'T INSTALL THIS HUD.YTD FILE, THEN YOU WILL NO NEW WEAPON ICON! Please do not comment on the 5-Mods page saying that you have issues when you didn't take the 5 minutes to read the clear instructions above and install the files properly...