Herd, 27, capped four times by Australia, was a free agent after cancelling a move to Perth Glory.
Nouble, 25, spent last season at Chinese side Tianjin Quanjian.
"In Frank, we've signed a strong, powerful and determined striker. Chris's versatility will be a big boost for us," said boss Justin Edinburgh.
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Gillingham have signed former Aston Villa midfielder Chris Herd until the end of the season and striker Frank Nouble on a short-term deal.