tf2 maps are broken due them having hl2 tex and not tf2 tex, this is because the textures use the same names as the hl2 ones, making the tf2 tex being hl2

the shotgun uses just the name "shotgun" and the hl2 one "shotgun" same with the tex this thing means the hl2 (being the first one you get) replaces the tf2 tex with the hl2 one

download this addon for the fixes
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=424885986&searchtext=tf2+fix
then you may go back

if your hl2 shotgun messed up, being invisble or being replaced with the tf2 tex, you may need to un-install the other addon and install this one =
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2148656795&searchtext=tf2+fix

to do this, you need to go like this (in the gmod spawn menu)
Games > team fortress 2 > weapons > c_models

then search for the folder called c_shotgun
and then move the shotgun to your weapon folder in the tf2 weapons (not in the game files in the q menu!)

to move it you need to hold your click on the model, you must scroll up to get to your tf2 folder, and then hold with the model in the weapons section, and then place it where ever in the other models of the weapons, you can also do this with the flamethrower since it isnt in the weapons section too