Travis’s Windows image comes with the “mingw” Chocolatey package, which may have the gfortran compiler.
Alternatively, you can uninstall the “mingw” Chocolatey package and install MSYS2 as shown here, then install the mingw-w64-x86_64-toolchain package group inside it with pacman -S. (You’ll have to use the mentioned trick to enter a MSYS2 shell from the Git Bash shell that your build starts in.)