The 27-year-old has agreed a three-month switch to the Priestfield Stadium.
Hollands was a member of the Charlton side which secured promotion from League One in 2012.
Gillingham boss Martin Allen told BBC Radio Kent: "He's had a great career and I've tried to get him many times.
"It's pleasing to get a hold of him after he's done very well at Charlton."
Gillingham have also extended Michael Harriman's loan switch from Queens Park Rangers until the end of the season.