Here's one of the info boards, it might actually be standing on top of a rampart:
http://en.wikipedia.org/wiki/File:Greystone_Farm_Nature_Reserve_-_geograph.org.uk_-_233609.jpg
The Gloucester Wildlife Trust website is not very informative about walks around the site unfortunately. I thought there might be a map of the permissive paths. There is this though:
http://archaeologydataservice.ac.uk/archives/view/salmonsbury_eh_2006/downloads.cfm