In a grave manner.
- Newage Dictionary DB (adverb) (adv.) (adverb)

Usage examples:

" You must, brother," said Haspinger, gravely
- Andreas Hofer by Lousia Muhlbach
" I think," he said gravely that it is a very serious matter."
- An Alabaster Box by Mary E. Wilkins Freeman and Florence Morse Kingsley
" Yes," Winthrop said gravely
- Hills of the Shatemuc by Susan Warner