That is so true. Oswald can get up there otherwise, but not when Mickey finally makes it. I presume you stood on the rock on the geyser, and thinned the board in front of the alcove. Ride the elevator up to the ledge where you entered from Angel Side. Thin the cactus closest to the alcove. Call Oswald and ride him over to the alcove. It may take several times, especially because the camera makes you fly blind, and you go down if you touch the wall.
Before you try to free the gremlin, the easiest way is to go through the area and paint in the skulls over all of the cracks in the ground where steam is rising. This will cause the pressure to build under the rock and it will raise all the way up and become level with the little area so you don't have to fight with Oswald to get up near the gremlin.