Please follow the steps below. But in order this to work, you need the version 6.4.0.117.
You can now override the minimum gift card length by entering the S5 setup registry with
LEVEL1=GIFTCARD
LEVEL2=MINLENGTH
INDEX=0
DATATYPE=wwInteger2
DataLen=2
DataDec=0
and set the value to 5.
Default is 6
Restart of System Five is required.
See screenshot below.