A minimal, customizable, ready-to-compile boilerplate for Game Boy RGBDS projects.
You can simply clone the repository using Git, or if you just want to download this, click the Clone or download button up and to the right of this. This repo is also usable as a GitHub template for creating new repositories.
Make sure you have RGBDS, at least version 0.4.0, and GNU Make installed. Python 3 is required for the PB16 compressor bundled as a usage example, but that script is optional.
Edit project.mk to customize most things specific to the project (like the game name, file name and extension, etc.). Everything has accompanying doc comments.
Everything in the src folder is the source, and can be freely modified however you want. The basic structure in place should hint you at how things are organized. If you want to create a new "module", you simply need to drop a .asm file in the src directory, name does not matter. All .asm files in that root directory will be individually compiled by RGBASM.
The file at src/res/build_date.asm is compiled individually to include a build date in your ROM. Always comes in handy.
If you want to add resources, I recommend using the src/res folder. Add rules in the Makefile; an example is provided for compressing files using PB16 (a variation of PackBits).
Simply open you favorite command prompt / terminal, place yourself in this directory (the one the Makefile is located in), and run the command make. This should create a bunch of things, including the output in the bin folder.
If you get errors that you don't understand, try running make clean. If that gives the same error, try deleting the deps folder. If that still doesn't work, try deleting the bin and obj folders as well. If that still doesn't work, you probably did something wrong yourself.
If you want something less barebones, already including some "base" code, check out gb-starter-kit.
Perhaps a gbdev style guide may be of interest to you?
I recommend the BGB emulator for developing ROMs on Windows and, via Wine, Linux and macOS (64-bit build available for Catalina). SameBoy is more accurate, but has a much worse interface except on macOS.