Gcaba has spent the past six months on loan at NFD-bound Platinum Stars after falling out of favour at Pirates, who have decided against exercising an option to extend his expiring contract. He initially joined the Sea Robbers from Free State Stars in July 2011.
Mthiyane is also on the way out at Mayfair, having failed to establish himself at the club since his July 2016 signing from Chippa United.
The Milutin Sredojevic-led Buccaneers are expected to announce some new signings in the coming weeks, with the likes of Paseka Mako, Vincent Pule, Brilliant Khuzwayo and Ben Motshwari all being linked with the Soweto giants.
Bucs earned qualification to the CAF Champions League next season with a guaranteed runners-up finish behind Mamelodi Sundowns in the Absa Premiership this campaign.
Sredojevic's men will conclude 2017/18 against Free State Stars at Orlando Stadium on Saturday (15h00).