slander, to revile, huge, big
謷 centers on harsh, loud speech: slandering or reviling someone. The meanings 'huge' and 'big' are likely rare or archaic extensions, perhaps from the idea of loud, overbearing talk.
謷 combines 敖 (which may suggest loudness or arrogance) with 言 (speech), pointing to loud, slanderous talk. The exact historical development is uncertain.
On top, 敖 looks like a person with a big mouth (the 方 and 攵 parts suggest action). Below, 言 is speech. Imagine someone shouting slander with a huge, loud voice.
For ギョウ, picture a slanderer making a loud, exaggerated 'gyow!' sound like a cat's yowl: gyow -> ギョウ, and the noise is as big and harsh as the slander.