1] No.
2] It's essentially an ongoing tournament. You only play people in your GRADE (S/G1/2/3). Everytime you win a match you gain your opponents GP and CP and at the end of the final you get +GP. G3 tends to work at 1/50/105~. G2 works like 0/1/50/105 GP.
3] It means you're at the bottom of the G3 bracket, which is E. IN order to rank up you have to earn more GP.
4] Not sure but on the PS3 it's in Player Stats from the main menu.