Tanjung Gemok, located near Rompin, about 20-30 minutes north of Mersing, is considered Tioman's secondary ferry harbour. Tanjung Gemok is a deep-water harbour and, as such, can be used to get to Tioman on days when the tide in Mersing isn't suitable.
If you are driving to Tanjung Gemuk from Kuala Lumpur, follow Kuala Pilah, followed by Muadzam Shah and then drive along the coastal road till you reach Tanjung Gemuk.
If you're coming in from Singapore, just keep going towards Mersing until you see Tanjung Gemok. The drive from Singapore to Tanjung Gemok is generally slightly longer than to Mersing. But the upside is that the ferry crossing from Tanjung Gemok to Tioman can be a little shorter in duration, although the difference isn't that big.
All above being said, Tanjung Gemok is a little less acessible than Mersing and lacks the generous facilities and amenities that Mersing does offer, including secure parking, decent eateries and a retail area. It also tends to be harder to get a bus or a taxi at Tanjung Gemuk than at Mersing.
In other words, a case of pros and cons. So best weigh these before deciding which harbour, Mersing or Tanjung Gemuk, you want to go.