I have had a machine put on my Citroen C4 grand Picasso and it's come up with error codes P0925 & P0919 saying gear shift error, does this mean I need a new gear box?

Code p 0919 refers to gear shift position control===causes wiring--- poor connections ---ECM---TCM
code p 0925 refers to gear shift reverse actuator --circuit /range performance ===causes --wiring---poor connection---gear shift reverse actuator---ECM---TCM
code p 1351 refers to IDM input circuit malfunction---ignition coil control , high voltage
there is nothing there to indicate a new transmission is required as the TCM ( transmission control module ) and reverse actuator or normally add on units and are separate to the box main
the wiring is always suspect as it hardens and breaks or the connections get corroded and do not pass power properly
I would at this stage not consider a new transmission and if it is suggested , I would get a second opinion from an accredit auto transmission specialist shop with experience on your vehicle