orbital diagram for Au is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 4f14 5d10 6s1
you just have to fill the "boxes" with arrows, s orbital has only one box with two arrows; p has 3orbitals with 6arrows; d has 5 boxes with 10 arrows and f has 7boxes with 14 arrows. In each box can be two arrows with opposite spin maximum!
you can have a look here, how to fill the orbitals: http://www.chem.uky.edu/courses/che105/1%E2%80%A6
Since Au+ has lost one electron and the numbers (or arrows in the diagram) represent the electrons, its configuration is: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 4f14 5d10. So there will not be the 6s orbital.
Since all of the orbitals are filled with electrons (i.e., there are no unpaired electrons in Au+ orbital diagram), then the ion is diamagnetic.
Hope this helps :)