Thanks! 

Bus gain is. automateable.

The compressor's makeup gain will bring up the low levels to some degree, which is another form of "glue" as it produces more consistency in the final output.
 
No one has defined the technical process that creates glue, but sonically, what it sounds like to me is that gentle compression "fills in the cracks" between levels otherwise wider levels.

To me it just sounds like it's tucking everything in a little and creating a little rhythmic "bounce" which makes the tracks sound like they're in step with each other. Very hard to put your finger on definitely though.