GAPS was the most common way to protect floppy disks against copy on Amstrad CPC. It consists of writing specific values (other than the standard &4E) in the separation area between 2 consecutive sectors.
When the protected program is launched, it checks if these special values (most often a string signature) are present. If they aren't, the program can then crash or reset the computer or format the disk.
The trick is that the 765 FDC can read the custom byte values in the gaps, but it cannot write them. This made it hard for people to make working copies of protected disks.