I live in Lovell wy and anywhere I put my radioshack pro 106 it has interferenceses. unless I put the GA on does that like only bring in the really strong signals? I've read about it but doesn't really make sense.
The GRE built scanners are known for their selectivity, they get over loaded easy by close signals. I have a few RS scanners myself and the GRE's all have this issue except my older units like the 2035 and 2006. The ATT trims 10db off (the GA) or in other words attenuates the signal received.
You can set the Attenuator for certain frequencies (overloaded freqs) and leave the A off on others for far away signals. If you have the Global Attenuator (GA) on then all frequencies are attenuated. Only the A will show if you set the attenuator for certain frequencies.