Hi there! You can still create an RProject and associate it with your .Rmd file. Simply place your .Rmd and associated files into a directory of your choice, then go to File > New Project > Existing Directory, and select the directory that contains your files. Once the project is initialized, you can go to Tools > Version Control > Project Setup, select "Git", and allow RStudio to initialize the Git repo and restart R. Then the Git tab will be available to you.
I hope this helps! If not, can you please clarify your question?
Thanks! Yes - it worked for one .Rmd file and not the other. I must have something else going on. The project file shows up under the Git tab but not the other files in the directory.