gmake is just another name for GNU make - which is the make program you already have installed. Years ago, you used to be able to run it as "make" or "gmake". Nowadays, I think they've stopped using the "gmake" name, you just run it as "make".
So as I see it, you have three relatively trivial choices:
1. Edit your script to change all instances of gmake to make.
2. Edit your .bashrc to add an alias
3. Create a symbolic link called gmake that points to the make executable.
sudo ln -s -T /usr/bin/make /usr/bin/gmake
Any one of those three options should solve your problem.
Personally, I'd use Option 1 and just edit the script to use "make" instead of "gmake"!
Option 2 adds a symbolic link called "gmake", which will allow you to type "gmake", or use "gmake" in your scripts and will run the "make" command.
Option 3 uses an alias to do pretty much the same thing as Option 2
NOTE: If you choose to use Option 2 and add an alias to .bashrc, the alias will be available in any new terminals you open up.
If you have any terminals already open, you can run the following command in each open terminal, to reload your .bashrc and make the alias available: