How can one determine the number of bytes in the SEGD Revision1 interblock gap?
The quantity is present on page 5 of the standards document.
There are no bytes in an IBG. SEGDrev1 is a tape format, and the schematic you are referencing in the standards document is a map of how the data is physically stored on a tape. Tape gaps are actual physical spaces on the tape between blocks of data. An IBG is, quite literally, a gap between blocks of actual data.
If you are reading a tape, you should be able to advance the tape to the next block as needed. If you are working with a SEGD file that has been copied to a hard disk, then gaps are nonexistent and you can pretend they are not there.