Gandalf a "steward?"

[davem]

'Steward' in this context seems equivalent to 'Thain' in the Shire: 'They chose from their own chiefs a Thain to hold the authority of the king that was gone.' (LotR Prologue).

The section in italics makes it clear that the Thain was to all intents & purposes in the position of the King, with all his power & authority. The Steward is in the same position in Gondor - unless the King returns. Denethor, therefore is not simply a 'servant' - he is 'King'. As is the Thain in the Shire. Gandalf is 'Steward' in that he is the representative of the Valar (possibly of Eru, rather, as even though it was the Valar who sent him to Middle-earth, it seems from his words to Aragorn/Gimli/Legolas that he had strayed beyond thought & time - ie beyond/outside Arda itself - that it was He who sent him back). It seems what Gandalf is saying is that while Denethor may be Steward/representative of the political & temporal ruler of M-e, he, Gandalf, is Steward/representative of the Divine ruler of all things. Its the old division of Church & State thing.