Most values in the ROM header are only cosmetic. The bare minimum requirements for a workable image are checksums, the Nintendo logo, and (if needed) the CGB/SGB flags. It is a good idea to pad the image to a valid size as well (“valid” meaning a multiple of 32KiB).
The following will make a plain, no-color Game Boy game without checking for a valid size:
$ rgbfix -v foo.gb
The following will make a SGB-enabled, color-enabled game with a title of “foobar”, and pad it to a multiple of 32KiB. (The Game Boy itself does not use the title, but some emulators or ROM managers might.)
$ rgbfix -vcs -l 0x33 -p 0 -t foobar baz.gb
The following will duplicate the header (sans global checksum) of the game “Survival Kids”:
$ rgbfix -cjsv -k A4 -l 0x33 -m 0x1B -p 0xFF -r 3 -t SURVIVALKIDAVKE SurvivalKids.gbc