Guncraft gives you the option to create your own custom skins. As long as it does not have an art issue or use copyrighted images and/or offensive language/pictures, it should be accepted. It takes one developer or 15 users to accept a skin.
Step one: Download template, resize and draw skin Edit
Firstly, get a Skin template by going to foundry and selecting custom skins, then download the skin template. (Pro Tip: Firstly resize the image to 128x128 and save it as another image, thus you do not lose detail while resizing.) Final templates must be 128x128 even though the one they give you is 512x512. Draw your skin.
Step two: Make a stencil file Edit
Guncraft uses stencil files to determine the colour of your skin when you are in the red or blue team. Copy your template you just made. Now edit it by making everything black that you want to stay the same and make everything white that you want to be the team's colour. Your stencil file should only be black and white.
Step three: Upload skin and choose sound file Edit
After making your two template files, you can navigate your way to foundry from the main menu and then choose custom skins. Upload your skin file and the stencil file. On the left handside you should see your player model, with the colours correctly applied. Choose your sound file and a name for your skin and submit for approval. You'll receive a notification in 1-2 days time if your skin has been accepted.