Does GTP-γS (GTP gamma S) bind all GTP-binding proteins?
This is the figure the question is about. On the right is the control experiment with GTP-γS, on the left without it: 



The bands that are visible in both experiments are unspecific binding. If GTP-γS doesn't affect their presence, the mechanism by which they bind to the column can't be specific to the GTPase functionality.

The proteins the authors were after are those that are present in the experiment without GTP-γS, but are not eluted in the experiment with GTP-γS. Those are binding via an GTPase functionality to the GTP on the column. They are marked with arrow heads in the figure.

The general idea behind this control experiment is to distinguish specific binding from unspecific binding to the column. Being able to do that means that the authors had fewer proteins they needed to investigate further, saving them some work.