The story
Blend of whimsical + goth. The earliest known use of the term is a tweet posted on August 6, 2020 by Twitter user @KaiserBeamz (see quotation below), possibly as a back-formation from whimsigothic (itself from around April 2020). The aesthetic was originally referred to "Whimsical Mystical Gothic Celestial", a term which may have originated in an image posted on April 10, 2020, also by @KaiserBeamz.
Etymology from Wiktionary, CC BY-SA 4.0.