Use OpenIV (with a mods folder) in "Edit Mode" then install to Grand Theft Auto V \ update \ update.rpf \ common \ data \ ai \ and replace your current "weapons.meta" with the one from this folder

The changes made to this weapons.meta will make sure that the Iron Sights of the model are properly aligned with the in-game camera, this is especially noticeable when you scope/zoom-in when in First Person mode. Everything else within the file is entirely vanilla GTA V and unchanged.
While installing this file isn't 100% required for the mod to work per say, it is recommended to install it anyway if you want to have your Pistol sights function properly! 
Sadly, it is still impossible to override entries from the vanilla, update.rpf's weapons.meta file via standalone add-on dlcpacks, so for now this is the only real way for it to work. 

If you already have a modified weapons.meta file used for other weapon mods however, then simply use OpenIV while in Edit Mode, find your current weapons.meta file which you are primarily using, right click then hit "Edit",
then once the file is open, find the following lines under the "WEAPON_PISTOL" main string of text, and replace them with the ones shown here:

          <FirstPersonScopeOffset x="0.0000" y="0.0000" z="0.0021" />

Then it save, and exit OpenIV. If you did that correctly, then you should have working Iron Sights for my Glock mod, with all your other vanilla weapon replace mods working as normal!

IF YOU CHOSE NOT TO INSTALL THIS WEAPONS.META, THEN THE SIGHTS WILL NOT BE ALIGNED PROPERLY! Please do not comment on the 5-Mods page saying that you have issues with this when you didn't take the 5 minutes to read the clear instructions above and install the files properly...