on the 172  stec 55 AP you hit NAV twice and a little banner lights up that says GPSS......but if I want to do a simple gps flight and approach

with the 530 I only have to have the GPS/VLOC in GPS and the NAV selected on the STEC 55 and that will work, yes????

I mean you don't have to have the GPSS banner lit up also????yes/no??? any info appreciated...

I'm not really certain of your question?  If you have a GPS flightplan you'd want the CDI to be in GPS (GNSS is the old world).  When you want to begin an approach, assuming it is an RNAV(GPS) approach, the CDI should still be in GPS and then you press APR or APPR.  The plane may also descend on a vertical glide path if the approach has vertical minimums such as LPV.  

If the approach you loaded is an ILS you'll need the CDI to be in VLOC, and have the correct localizer frequency tuned into NAV1, before you press APR mode.

I went the G1000 anyway so it doesn't matter...