Gibberish, mummery, hocus-pocus, abracadabra are comparable as terms of contempt applied to something which is in itself unintelligible or meaningless to the person concerned. They are often used interchangeably but are not true synonyms.
Gibberish suggests language; it is applied especially to inarticulate but expressive sounds or attempts at speech. When applied to articulate but unintelligible utterance it may imply a low-grade or disordered intelligence in the speaker, or it may suggest jargon. One or more of these implications is carried over when the word is used to express contempt.
Mummery suggests not language but actions uninterpreted by words (as in the old dumb shows). As a term of derogation, however, it is applied chiefly to rites, proceedings, and performances which, whether or not accompanied by words, appear theatrical and ridiculous as well as meaningless to the observer.
Hocus-pocus suggests jugglery and incantations. Sometimes, in its extended use, the stress is placed upon tricks intended to mystify or confuse, sometimes upon empty but impressive-sounding words.
Abracadabra basically applies to a magical formula or a mystical figure and in extended use is applied chiefly to discourse and implies not only its unintelligibility and formulism but its complete unfitness for the ends it proposes to achieve.