I am not sure you can add a pause in the Prefix per se, but you can increase the Intercharacter delay which should achieve the result you want.
Download the Product Reference Guide for the GM4100 from this page: Gryphon GD4100
Here is the direct link to the 13MB PDF:
You will need to print several pages so that you can scan the appropriate codes to change the scanner setup. Follow the instructions on the following pages:
Pages 47 and 292 describes the Intercharacter Delay.
Pages 351 and 352 in Appendix D give you the number scans you will also need.
You might start with a 10ms delay and increase this until it works. A little experimentation will surely solve your problem.