If what you're trying to do is display the keyswitch mode in an HMI, you're going to find out in a bit that of course the GSV doesn't execute when you're in PROG or REM PROG, so you can't use it to determine those two modes.
RSLinx Classic and RSLinx Enterprise both include "pre-defined items" that directly query the controller's Mode Object and will tell you what mode the controller is in. Look up "@Mode" or "Pre-Defined Item" in the RSLinx online help or the Knowledgebase.
Any other HMI is going to need a generic CIP Object query (it's not a Tag that you can read with an ordinary Tag Read service) and I haven't seen any that support that function.