One Multi-OS Swift single Gitlab repository project

A single Gitlab resource for macOS, Linux and Windows?

I am developing a new linguistic engine, capable of processing over 12 types of linguistic phenomena (ranging from spelling, morphology, syntax and semantics) for any written natural language. I use Xcode/Swift on a Mac; but there are Swift compilers for Windows and Linux as well. I would like to manage only one single Gitlab repository for my source (I do not want to duplicate my source files) that could be downloaded, edited and compiled by users on Windows or Linux machines.

How can I store in one same repository (= git-ed folder structure?) lib and executable files for 3 different OSs? Any advice?