Hello Dusan
First go for a long ride to use most of the fuel. The tanks b....y heavy when full. Remove seat. Disconnect battery. Remove plastic side cover on right side. Disconect fuel feeds, overflows and wiring connector and remove tank. Remove live terminal on starter motor cover on left, it unscrews just liek any other bolt. Remove plastic starter motor cover. One screw at rear and the gently pull it backwards. The front edge is located on two lugs which locate in rubber bushes. Remove sump bash plate. Remove exhaust down pipes. Loosen silencer fixing to hanger but leave in place. Trace wiring from sensor in catalyst to plug on right side of frame below tank location, unplug and cut all ties. Unthread wiring and lay in safe position. Remove fixings for catalyst each side by centre stand pivot. Working from left remove silencer fixing and remove silencer and catalyst complete, protect trailing cable. Try not to give yourself a hernia as you realise why your beloved GS weighs 250Kg wet. On back of gearbox in front of swing arm you will see a black rubber item held in place by a spring clip This is the gear position switch and the apparent cause of woe. Ensure area is clean. Squeeze two ends of clip together and slide sensor backwards from rear of box. Trace wiring behind starter motor and up left side of frame to plug & socket by frame rail beside the ABS units or the space where they would be if you have a non-abs bike. You may need to remove the plastic liner from the rear wheel arch and loosen or remove the plastic box that contains fuses, relays etc. to trace and remove the wire for the switch. Disconnect the plug/socket with care as the terminals inside are very small. Remove wiring and switch, get the new one and reverse the above procedure. I would suggest connecting the new switch prior to installation and reconnecting the battery to see if everything functions correctly. You can puta screwdriver into the slot on the switch to rotate it and simulate the gear changes. Be very gentle as I don't know how fragile these are. You can also move the gearlever and check that the spindle protruding through the gearbox housing to activate the switch is moving.Then reverse the dismantling precedure.
do you suffer from nightmares 
