Hi Pete -
Yes, a Compressor will lower the level of the louder parts of the track [levels above the Threshold]—in the GB Smart Control panel ["B"] you'd set the Threshold [labeled Amount] so the Compressor kicks in only on the louder notes [using the LED meter as a reference]; if you open the Compressor plug-in window you'll see that the Amount control actually is the Compressor Threshold control, and you'll have additional controls—Ratio determines how aggressive the compression is [typical settings might be around 3:1 or 4:1, higher for sharp percussive instruments like drums or slap bass], and [makeup] Gain sets the appropriate overall level for the track. Within the plug-in window you'll also find some presets for different instruments, which you could call up to get a feel for appropriate settings—but keep in mind Threshold must always be set by hand for the current track.
This video covers the basics of compression—it shows the Compressor plug-in in Logic, but the information applies to any compressor [and remember, GB is really a simplified version of Logic under the hood]..