What does mean 'GS' in RNAV (GPS) Chart?
Are you asking about the specific term 'GS'?  I don't see that on the chart in your question.

The angle symbol and the 7.75 are explained as a Visual Descent Angle in the Chart Users Guide.


  A VDA and TCH may be published on non-precision approaches. For Copter approach procedures, a Heliport Crossing
  Height (HCH) will be depicted in place of the TCH. The VDA is strictly advisory and provides a means to establish a stabilized descent to the MDA. 




So it is an aid for maintaining a stabilized approach that should put you at the MDA at the correct point.  


  If the last one's word is true, an airport have a 7.75 GS, like a KSBS Airport, then the PAPI of KSBS airport indicates 2white-2red when I follow the 7.75 GS ?


No.  In fact it's called out on the chart: "VGSI (Visual Glide Slope Indicator) and descent angles not coincident".  So they do not match.  You can look in the chart supplement for the airport and see what the actual angle is.  For KSBS runway 32, the Glide Angle (GA) is 4 degrees.


  RWY 14–32: H4452X100 (ASPH–GRVD) S–50, D–60 HIRL
  
  RWY 14: Rgt tfc.
  
  RWY 32: REIL. PAPI(P2L)—GA 4.0º TCH 40´. Thld dsplcd 600´.