How to handle scss and css in git?

You probably should not store generated files in git. In this case, the generated files would be the .css and .css.map files that compass (I'm assuming you use compass because of the tags) generates for you. You should store the .scss file in source control, and compile it to .css afterwards.

If the freelancers are making direct manual changes to the .css files, they should know that at each recompile those changes will be lost.

The .css file is generated from the .scss files. Your developers should commit only in the .scss files. In the most cases the .css file is not added to the repository at all. And you should not modify the .css file directly because it will be replaced with the next compilation of .scss the files.

GULP is just the tool that compiles the .scss files and create the css from them. Basically when using GULP you can create some functions where you can specify the input location(.scss), output location(.css) and additional rules, etc.

There are also other tools that can do this. Like Koala, Webpack.