We use a GS15plus and it has given amazing service over many many years, I am not sure but I think all Gs15 are tangential and that the swivel only knife version was actually called the GSX15, anyway I have the same manual as shown above can't help you with the hidden button, remember to oil every six months as per page 51 service and maintenace. As it says don't get oil on the belt, I did (lots of oil) and had to change a belt, not difficult but hard to source. Use 3 in 1 oil or sewing machine oil.
I have changed the belt on mine and done several minor repairs the machine is basically bulletproof. good luck
Tel