Gimli and the Steward's Chair

[Bombadil]

I believe it was just a flat out sign of disrespect. Estel mentions before how Gandalf respected Denethor, which is true, but I could not see any repect from Mithrandir in the movies.

Ever since TTT Gimli basically became a source of comic relief for viewers, which is too bad- yet his character was still developed as a hot-headed dwarf who strayed from being polite at times. It was rather fitting at that point to show the careless dwarf disrepecting the Stewardship of Gondor. (btw all my dwarf descriptions taken through movie characterization, not book!)

Of course before long Gimli came out with another comical line while in the chair, to further add to Jackson's characterization.