As Nabeet says. It is Generic Object Services. It's been around for some time and is really handy. I've used it for links to IDOCs and Workflow, as well as attaching documents against Business Objects.
The best place I found for info was at this link on SCN
http://scn.sap.com/docs/DOC-8327
The following two links in the doc are two of the most interesting ones:
- This link in SAP Help explains how to add the button to your custom transaction
http://help.sap.com/saphelp_nw70/helpdata/EN/94/aa532cddd511d289860000e8216438/frameset.htm
- How to Maintain GOS Service List & Toolbar is really useful.
Have fun developing and enhancing your user's experience...