[Rhod the Red]

Denethor, when growing up before becoming Steward, was disturbed at the 'usurpurs' of Gandalf and Saruman visiting Minas Tirith and advising his father on the policies of Gondor.

You will note that in the Return of the King he mentiones his view that "there can be no greater purpose than the good of Gondor". He saw himself as rising to the position and then defending Gondor's sovreignty against both Mordor and the Israri.

So it was straight power and poltical reasons he disliked Gandalf.