The LNAV and VNAV buttons are assigned to the FMC (Flight Management Computer) which you control with the CDU (Computer Display Unit). I have found the MSFS GPS system to be inferior to the FMS when it comes to flying jets. It does not take into account speed, or altitude during course changes, plus it does not give any vertical guidance. There are no SIDS or STARS in the MSFS GPS. Use the CDU, its how the plane was meant to be flown.