Ghouls are fully material.
The Monster Manual entry for the ghoul does not make any mention of other planes, so they are on the material plane by default. In contrast, the entry for the ghost specifies that
The ghost enters the Ethereal Plane from the Material Plane, or vice versa. It is visible on the Material Plane while it is in the Border Ethereal, and vice versa, yet it can't affect or be affected by anything on the other plane.
Generally, without a passage like this in the MM, we can assume that the monster is fully material.
As a DM, you have the power to change this.
DMs have the final say on everything in the game; after all, this is your unique world that you're creating, and you're not bound by any canon (unless you want to be). For example, you can decide that ghosts only exist on the material plane. Alternatively, you can say that a particular ghoul comes from another plane and is intangible on the material plane.
Of course, you'll have to communicate such changes to your players somehow (via knowledge checks or by having them interact with the creature), and you might have to rebalance the creature if there's going to be a fight.
I personally do this quite frequently, mashing together monsters or changing key parts of their powers, if only because I know that some players are familiar with the monster manual and I want to shake things up a bit.