Well, according to LOTR Appendix, Gil-galad was High King of all the Elves of Middle-earth. I always thought this factiod worthy of debate since during the First Age, no such title existed. There was a high king of the Noldor but not a High King of Elves of ME. It's all the more surprising that this status was apparently even recognized by the Sindarin/Silvan realm of Greenwood. I remember reading somewhere that the Sindar who emingrated east of the Misty's were particularly anti-Noldor in their sentiment so it struck me as odd their willingness to submit, even theoretically, to Gil-galad.

Although this is the subject of debate, I've opined that Elrond would have been entitled to assume the title (high) king of the Sindar as Thingol's heir via Elwing and Dior. Elrond had dual nobility, also being of the Noldorin royal house (although not in the line of succession) via Earendil and Idril. The only known concentration of Sindar in Gil-galad's core realm was in South Lindon and they were ruled, for a time at least, by Celeborn asa vassal of Gil-galad.

Eregion, as a Noldorin realm would in theory be sunject to Gil-galad but he did not seem to exercise any real authority over Celebrimbor. Had he been able to exercise such authority, he should should have been able to order Celebrimbor to break all contact with Annatar/Sauron.

Thus, the High Kingship of Gil-galad appears to be somewhat of an honorary title. I see no evidence that Gil-galad was able to tell anybody to do anything. The other Elven realms did respond to his call for the Last Alliance, but I don't get the impression they felt under any sort of legal obligation to do so. Greenwood and Lorien joined because it was the right thing - morally and strategically - to do, not because Gil-galad told them to.

This also appeared to be the case of the First Age high kingship of the Noldor in Beleriand. The supposed high-kings were not great leaders and oftentimes allowed Maedhros and the Feanoreans to take the lead. Orodreth refused to commit Nargothrond to the fight even though the High King was involved. Apparently, the High King could not order another king field his army.

Gil-galad's relationship with Numenor waxed and waned. Gil-galad was oftentimes in desperate straits and the Numenoreans came to pull his fat fromt he fire in the War of the Elves and Sauron. By the end of the Second Age, obviously the official policy of Numenor was anti-elf but good relations were kept up by the Faithful - which relationship would eventually bear fruit with the Last Alliance.

In the Third Age, the Elves had a chance to repay the favor, helping out Arnor various times it its hours of need.

Russ