Midfielder Hollands has managed just one Capital One Cup start for the Addicks this term but has racked up 62 appearances for the Championship club in total since joining two years ago - winning the League One title during his first season at The Valley.
The 27-year-old - who started his career with Chelsea - also spent two months on loan at Swindon last term, scoring twice in 10 outings.
Meanwhile, right-back Harriman will now remain at Priestfield for the rest of the campaign after the Gills agreed an extended stay with parent club QPR.
Harriman has started all four matches since joining Martin Allen's side on a one-month deal at the start of August and will act as a replacement for Matt Fish, who is sidelined long-term following his ruptured cruciate ligament.
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