Gecko supports the character encodings defined in the Encoding Standard. Additionally, for the time being, Gecko also supports Big5-HKSCS as a distinct encoding available to Web content, because Gecko's Big5 implementation does not yet comply with the Encoding Standard (which unifies Big5 and Big5-HKSCS).
In Gecko-internal APIs, it is necessary to use particular (potentially) mixed-case names for the encodings. For encodings that have a compatibility name in the DOM Standard, except gbk, this is the compatibilty name. For other encodings, including gbk, this is the lower-case name from the Encoding Standard.