From GunBrokers web site:

"You can add to the description of your item at any time by navigating to the For Sellers page and selecting the 'Add to Description' function. You can add HTML, text, or pictures to your item listing. Javascript may not be inserted. Any addition made to the item description will be clearly marked with the date and time of the addition."

I have added to a listing after a bid but it was not easy. I ended up emailing Gunbroker and was walked through the process. Perhaps it has recently changed.