I see that you're a young chap trying to understand these systems. I'll try to answer your questions simply.
1) The FMGS sends signals to the FD which then sends it to the A/P and then to the computers, right?
Very basically yes. Although it's a more complicated than that in reality.
2) So do the computer calculate pitch and bank angle or is it the FD computer itself xalculating it?
The FMGS is (very simplified) a FD and AP in one. It computes a demand signal required to fly the path requested and sends it to the flight control computers. The pitch and bank required to do this is calculated by the FMGS.
3) So are the GPS antenna connected with FMGS so that it gives the right info to the FD? Are the GPS connected with other systems in the cockpit?
GPS antenna is not connected to the FMGS. It is connected to a GPS receiver (sometimes called an MMR). Raw GPS signals will only provide PPOS. There is no guidance. The GPS PPOS from the MMR is fed to several boxes in the aircraft
4) If we lose FMGS, do we lose A/P managed part?
Yes, if you lose the FMGS you lose the autopilot.
5) Even if we have lost FMGS, A/P is still using GPS for our selective guidance part, right?
See answer 4
6) If we lose FMGS and GPS, INS/IRS, can the plane still automatically follow a CRS? A HDG?
See answer 4. In addition if you lose the INS/IRS you will also lose the autopilot as IRS provides the attitude and heading information to the FMGS.
Hope this helps?