well, when you put it that way, if they were pitted in combat, Mr. Strange (I don't know who he is. I am trusting you're description) would more than likely win. If he is immortal, he can never truly die, making it impossible for gaara to win from death of his opponent. He can easily keep a fair distance from gaara and use magical attacks from a distance. Gaara is, as said in another answer, weaker and no longer has some abilities because of the extraction of the ichibi (Shukaku), although Gaara still has the deadly sand coffin and Sand burial.
Although it will be difficult, gaara has a chance to physically disable Dr. Strange much like what Shikamaru did to Hidan. He can create a sharp blade of extremely dense sand (as hard as the sphere gaara created when he first fought Sasuke in the Chuunin exam arc.). Using the blade, he can seperate Dr. Strange from his head. Dr. Strange is still unable to die, but I figure that when the head is separated from the body, the body will be unable to move.
So Gaara has a chance of winning, but he will have to think up of this particular startegy or another strategy to defeat this Dr. Strange. But Dr. Strange has great advantage over Gaara (according to your description.).