Well, it adds interesting features. In Zombie U it's your map, radar, and inventory control. In Lego City Undercover, it's your map GPS, communicator, police scanner. In Need for Speed Most Wanted the 2nd player can use it to control the weather and add or remove traffic when player one is running through the city. In New Super Mario you can have 4 players on screen and a the gamepad can act as a helper by stunning enemies and adding blocks to help access high platforms and protect everyone from falling.
So it depends on the game but when used properly it's a fantastic way to play.
"Do not be to eager to deal out death and judgment, for even the very wise cannot see all ends." -Gandalf
The panorama apps and Nintendo Land's plaza are pretty cool immersion wise, but really its the asymmetrical local multiplayer that is more impressive. I don't think immersion is the purpose. It is more natural like ipad motion controls than immersive like Wii Motion plus motion controls.
It is also nice for off screen play with the built in TV remote for fast switching, but I'm a husband and dad in a limited household space, so that might not be a huge thing for others like it is for me.
On a side note it is totally immersive with it's use in Zombie U. Holy crap that game gets intense.
To be honest the answer as of now is no. Why? Because there aren't enough games on the Wii. The potential is there and is used AMAZINGLY on nintendo land but there just aren't enough games that make use of it. So no it's not gimmicky, it can get really deep but it's just that their aren't enough games.
I don't need violent videogames to reassure myself that I'm an adult.
At the moment it feels like neither. I love the gamepad and it in no way feels gimmicky to me, but I also don't think it's made any game I've played on the Wii U feel more "immersive" (except maybe Nintendoland - the motion controls are clearly much much better in the gamepad than on the Wiimote). It certainly hasn't distracted like some claim.