There is actually a way to find that closed Gunbroker auction, but it requires what is called "The Wayback machine" which basically archives every page of the internet that the spider has crawled.

I did a cursory search, but it is not very user friendly, but the data is probably in there somewhere....It's just having the correct date and search term and then hope that the search bot has archived that page.

Here's Gunbroker's wayback page if you to pour a stiff drink and give it a whirl... You can search by date and then time within that day when the bot crawled and archived the page:

Wayback Machine