How to reduce guard gap of a component in Proteus
Before trying to mess with the pad settings, you will want to look at the design rule manager. This can be found in the 'Technology' dropdown menu on the top of the screen, then click 'Design Rule Manager' 



Here you can change the clearance settings. You will want to change the Pad-Trace clearance. The default is 10th. 



If you make the gap too small in this menu, then it could be overruled by the pad settings itself, which will still flag a DRC error.

To change the pad settings, go to the PCB layout screen, you need to find the pad style in the left hand menu. Right click it and select 'Edit'.



You can change the guard gap here. The default for most is 5th, but you can change it to whatever you wish.