The Gary chip provides bus traffic control signals for the address and data bus. It was a consolidation of a large amount of discrete 74-series components and PALs originally in earlier Amiga models like the A1000.

Basically, it is needed for the 68k to even read and load the BIOS or access the ROM, so is a core chip needed for the most basic operations to work.

During normal operation, below is a basic description of how every single pin should be operating if probing using an oscilloscope.