You can spawn the weapons from Spawnlist menu. Some of them are not shown in there, so you need to browse them manually by choosing 'Browse' tab.
---------
Now, I will show a tut to make your own Spawnlist in GMOD in very detail way (I guess).
If any custom models don't have their own Spawnlist, we need to make one
for easier spawning, without having to get all through, browsing the models in 'Browse' tab.
Spawnlist is a text file that need to be place in 'settings' folder.
If you download custom models, whether from FPSBanana or Garrysmod.org, every ZIP/RAR files will contain these folders:
-Materials
-Model
These folders are common for skins installation, so make sure to keep them somewhere first.
Here, I show an example to install L4D2 survivors and infected model with its spawnlist.
* Now that you have extracted 'models' and 'materials' folders, first thing to do is create a New folder, lets say '
Left 4 Dead 2 Survivors and Infected' and
consider it as a main folder.
Then, cut and paste 'models' and 'materials' into the main folder. Next, cut and paste the main folder into
steamapps/YOURNAME/garrysmod/garrysmod/addon

* In a main folder, create a new folder and name it 'settings'. Double-click the folder and again, create a new folder and name it 'spawnlist'
* In 'spawnlist' folder, create a New Text document and name it whatever you want. Mine is 'Left 4 Dead 2 Survivors and Infected.txt'
* Open that .txt document with Wordpad and paste this script texts (click the spoiler):
» Click to show Spoiler - click again to hide... «
"0"
{
"Information"
{
"name" "Left 4 Dead 2 Survivors and Infected"
}
"Entries"
{
"1"
{
"model" "models/survivors/survivor_coach.mdl"
}
"2"
{
"model" "models/survivors/survivor_gambler.mdl"
}
"3"
{
"model" "models/survivors/survivor_mechanic.mdl"
}
"4"
{
"model" "models/survivors/survivor_producer.mdl"
}
"5"
{
"model" "models/infected/boomer.mdl"
}
"6"
{
"model" "models/infected/boomette.mdl"
}
"7"
{
"model" "models/infected/charger.mdl"
}
"8"
{
"model" "models/infected/hulk.mdl"
}
"9"
{
"model" "models/infected/hunter.mdl"
}
"10"
{
"model" "models/infected/smoker.mdl"
}
"11"
{
"model" "models/infected/spitter.mdl"
}
"12"
{
"model" "models/infected/witch.mdl"
}
"13"
{
"model" "models/infected/jockey.mdl"
}
}
}

** Take note that this is a common spawnlist script and always been used for any custom models . It's better
to understand this format if want to create more spawnlist later on.
Before creating, you need to know how many models you need for spawnlist. In this case, L4D2 survivors and infected together have 13 models.
To check how many models for spawnlist,look back at your 'models' folder, sort the files by 'Type' and count how many models in there (model file has .MDL as
its extension).

* Save the text document.
* Finally, run the game, open the Spawn menu, choose Props tab and there..You have just created your own spawnlist!

I believe this method should be no problem since I used this method many times. There's actually a simple program tool to make your own GMOD Spawnlist,
but it seems that some people have a problem with it; spawnlist doesn't work. So, just use this 'hard, long way' to create a spawnlist.