Code:
DisposalCode(TotalFrames) = ((Asc(Mid$(sBuff, lngFind + 4, 1)) \ 4) And 3)
This takes a byte at the position lngFind+4, shifts it right by 2 bits and masks the last two bits.
The result of this operation can be a value from 0 to 3.
The value was contained there in a byte: xxxxccxx, where cc denote the bits of the disposal code.

I don't understand how you compute lngFind, because this value +4 makes the actual pointer to the byte read.
Unfortunately, I don't know the format of the gif files so I don't know right from wrong here, but I can definitely say,
the result of the above operation gives either 0, 1, or 2, or even 3 if that is a valid disposal code, too.