https://nesrocks.itch.io/naw (Windows)
The NES Assets Workshop tool aims to aid with the creation of graphical assets ready to use with NES development. It is meant for artists and technical artists, and it automatically imposes all NES limitations and gives you a WISIWYG workflow.
The tool is 100% made in GMS2 with GML and includes a lot of very low level bits and bytes handling to keep true to the real NES graphics formats.
It is in active development and offered as name your own price.
Feedback is appreciated. Thank you!