You found the Giants' Table, but the giants there attacked without warning. They have lost their respect for the old ways, and the Giants' Table is no longer a place of peaceful negotiation. You should return to Gwaemithrin with the news.
Notes
The scout Gwaemithrin is in the highlands north and east of Rivendell at a camp established by the Dwarves.
Loc is 29.1S 1.9E Just run/ride by the table and you get attacked by giants there at table, and quest completes.
Lot of elite trolls/giants all over this area. So for those at or below quest lvl, you need to be careful, but with care you can do this solo. Only agro, if you are careful, is at the table. A burg with stealth can avoid the agro at the table, possibly. For all others just be prepared to run after agro, have a path planned out.
You need to be solo for some reason, was helping a friend and he could not get the quest to update while he was in group.