Guri did take on Luke around the time of Episode Six, when he was a full Jedi ready to face Vader. And she did defeat him. She knocked him to the ground and was in the process of striking a killing blow when Luke usede the Force to regain his lightsaber. And if by "defeat" you mean "knock to the ground", she did the exact same thing to Luke seconds before.
A battle between Guri and Luke w/ his lightsaber never happened. But she did defeat him unarmed.
However, that really doesn't matter all too much.
Looking back on Guri's accomplishments, I think she'll stay to guard Xizor. She's an amazing body guard, able to hold off Hero Characters (Hero Characters!) such as Luke, Leia, Lando, and Chewbacca. Wether it's by throwing large chairs, sticking her fingers through skulls, or going toe-to-toe with Luke Skywalker, Guri is one impressive bodyguard. Xizor may not be a combat expert, but with Guri at his side he doesn't need to be.
Yes but Guri only brought him down that time, because he got distracted. Before that Luke used Force Speed to evade all of Guri's attacks. Take away the getting distracted part, him getting knocked down wouldn't have happened.
Using the Force, Skywalker was able to evade Guri's attacks. Momentarily losing his concentration, Skywalker was knocked to the floor by Guri who moved to strike a killing blow. With the help of the Force he was able to move away from her blow and used a leg sweep to send her to the floor, igniting his lightsaber.
But your right Guri would more then likely just stay to guard Xizor.