First, download the Workshop addons from Source Filmmaker that you want to also copy to Garry's Mod.

Hot Tip! Make sure the addons you want don't already exist in the Garry's Mod Workshop. Check the authors' Workshop pages, or perform a search by name in the Gmod Workshop search bar.

To find out where the SFM Workshop files are stored, launch SFM. Load a map, or not - it doesn't matter. Right-click in the Animation Set menu and choose Create Animation Set for New Model.

After it loads all the Workshop files, navigate to the appropriate subfolder, or filter it out by typing its name in the "Filter" search bar. Note the full path of the model.

All models will be stored in $STEAMFOLDER$/steamapps/common/SourceFilmmaker/game/workshop/models, and their corresponding materials in $STEAMFOLDER$/steamapps/common/SourceFilmmaker/game/workshop/materials/models. Remember the base folders where your model and its materials are stored.

If you search the internet, you may find some pretty cool models that are not available on the Workshop. Download a model that you like and check its content.

Here are some places to get you started:

• SFMLab[sfmlab.com] - Many models for Source Filmmaker that are not on the Workshop can be found here. If you need to see more, you can toggle the "Furry" switch to find anthros (like Sonic the Hedgehog characters). I recommend NOT toggling the NSFW switch - although you may have an easier time looking for models you can dress up, you will also run across some... erm, cringe. Unfortunately, SFMLab doesn't have better filtering to isolate SFW nude models apart from oversexualized ones, therefore if you must, proceed at your own risk. They also run Open3DLab and SmutBase (another site I'd avoid), but these models are not guaranteed to work in Garry's Mod or Source Filmmaker.
  
• DeviantArt[www.deviantart.com] - Unfortunately, there isn't a centralized location for all Source Filmmaker models. Your best bet is to either do a web search, or search directly on DeviantArt, with words like "SFM", "Source Filmmaker", "model", and "addon". Check the description of the model for a download link (usually Google Drive or MEGA), but be careful when downloading from Deviants with newer accounts, or simply largely unknown Deviants, as you could be downloading viruses. Also stay away of any marked NSFW or 18+, unless... you know, that's your thing. 😬
  
• GameBanana[gamebanana.com] - Not a whole lot of models that aren't already on the Workshop get ported here, but it's worth at least a look.

This is where it can get time consuming - you need to make sure that every capital letter in a file or folder name is converted to lowercase. Garry's Mod, being older than Source Filmmaker, sometimes doesn't take too well to uppercase letters, despite having been updated to a more recent version of the Source game engine, and thus some things are case-sensitive. This is especially true on Linux, but that may not be the case on Windows. However, if your model ends up broken or missing, do this step.

So, if you find something like...
...you need to rename it to...

Hot Tip! Be careful not to rename anything, besides replacing uppercase with lowercase, as this could break the model. Even if there are obvious misspellings, foreign words, etc., leave those things as they are - it's better to have improperly-named files and a working model rather than fixing the names and risk breaking something.

Create a folder with the name of the model somewhere outside of game files. Then, make a structure like so and copy the folders as follows:

Now, you can do one of the following:

• Copy both the materials and models folders and paste directly into $STEAMFOLDER$/steamapps/common/GarrysMod/garrysmod.
  
• Copy the folder named after your model and put it in $STEAMFOLDER$/steamapps/common/GarrysMod/garrysmod/addons.
  
• Drag the folder named after your model onto $STEAMFOLDER$/steamapps/common/GarrysMod/bin/gmad.exe, and then copy the resulting .gma file and paste it in the addons folder.

If you enjoyed this guide, give it a thumbs-up! If not... well, do what you must. Make sure to favorite this guide for easy referencing later.

If you absolutely have to fix the models you import from Source Filmmaker (more flexible posing, NPC/Playermodel conversion, etc.), then I recommend this guide from Frosty Dr. that should be quite easy, with the most basic Blender knowledge required. It's actually not super difficult to understand.

God bless you all, and happy modelling! 😎💖🙏🏻