The best tools for making pixel art PNGs like you would use with GB Studio are currently Aseprite on Windows and Linux, Pixquare on IOS, and Pixel Studio on mobile. This tool is for when you need to export your pixel art to .c specifically for use with GBDK (although I wouldn't be surprised if there are other graphical development kits it would be good for.)
Aseprite and Pixquare are paid software, but Aseprite you buy once and own not only the software but any updates they publish for life. Pixquare I believe is similar, but I haven't touched an Apple device in nearly ten years so I can't say for certain.
Pixel Studio is free with a paid option for ad free and premium features (including making pictures above a certain canvas size.)