Best way to start is to check supply voltages. If the cards are in a rack voltages should be the same on each board but the edge connectors will be old and may cause problems. If you don't have a card extender to bring the board out of the rack then you'll need to make up a couple of edge connectors on flying leads & connect then to the power supply
Then feed a test tone to two units, one working and one not, and trace the signal through the board comparing the two units. As soon as you find a difference between the two you're on the way to finding the problem
From my research the Gain Brain is a relatively simple transistor-based compressor using an FET for gain reduction. The schematic is confusing but most of it is metering. The actual active audio part is pretty small
Checking DC voltages across transistors may be instructive as you compare boards but be careful not to let your probe slip & short things
If you print out a schematic and mark measured DC voltages on it from a good board you may be able to jump to finding faults quite quickly
And, of course, replace all old electrolytics and tantalum caps...
Nick Froome