The "Lawn gnomes" were the former overlords of the Gnome Dimension in the Wii/PS3 version of "Phineas and Ferb: Across the 2nd Dimension (video game)". They are the most common enemy found in the dimension.
At some unknown point in the Gnome Dimension, the lawn gnomes managed to get a gem that gave them all mystical powers. With their new-found powers, they conquered the dimension and forced it's citizens, including Baljeet and Isabella, to wear the lawn gnomes outfit at all times.
Later, when the characters the player chose arrived at the location of the gnomes' source of power, the gnomes try one last time to defeat them. They fail, and the source of their power is destroyed.
Physically, they don't look different from the other lawn gnomes.
Abilities and skills
In general, these lawn gnomes have the ability to move around (doing so in a way that makes them wobble) and attack. The blue gnomes can only do that while the green gnomes can also fly and, with the help of a magic staff, shoot a single blast at the heroes.
After the destruction of the gem that gave them their power in the first place, they most likely no longer have these powers.