"standing wide open," 1570s (implied in gapingly), present-participle adjective from gape (v.).
Entries linking to gaping
gape v.
early 13c., from an unrecorded Old English word or else from Old Norse gapa "to open the mouth wide, gape" (see gap (n.)). Related: Gaped; gaping. As a noun, "act of opening the mouth," from 1530s.