The tilde means home directory so you just have to edit the file called `.profile` in your home directory and enter the PATH definition from either `.bashrc` or `.bash_profile` also in home.
In any case, if you don't wish to edit these files, as a temporary workaround, you can open the terminal app, type ``` bash ``` to be sure you use bash (Catalina switched the default shell to zsh) and then type ``` /Applications/Geany.app/Contents/MacOS/geany ``` The executed Geany should be able to execute the fortran compiler.
Being able to specify which shell to use for executing compilers would be the right solution but it will have to wait until the next Geany release.