Hallo, I have a G 5120 and this is my first guitar. The more I read here the more things fall on me and the more questions come to mind.
So, is this the right way for the bridge base to sit on the top?
The base only touched in the middle of the body, must it lie with the whole area on?

http://up.picr.de/7791698ycy.jpg

The bridge is "ok" like that, but the contact surface is much less then what it should be. Place some PSA sandpaper under the bridge on the archtop and sand it down back and until the bridge base matches your archtop. The best time to do this is when you change strings.

So you want to be a bit careful when you do this.
First you have to remove the strings.
Cut out a fairly large rectangle of sandpaper, wider than the bridge and maybe 3 inches long. I like to use electrical tape to tape it to the guitar top but first, I put some tape around the 4 edges of the backside of the sandpaper. This will protect the guitar top. I then tape the paper down over the spot where the bridge goes.
Take the bridge and in a gentle back and forth motion (that is, toward the headstock and then away from the headstock) pass the bridge over the sandpaper. More pressure will get the job done more quickly but don't be too impatient. It will take a little while.
Eventually, the feet of the base and the top will have the same contour.
Good luck.

Thank you for the good advice, I will do so the next days.