There are plenty of glute exercises you can do without having to bend your knees very far or putting too much pressure on your knees.
Stiff Legged Deadlift - Primarily a hamstring exercise, it also targets glutes and lower back.
Glute Bridge - Can also be done with as a Barbell Glute Bridge, or done with feet elevated on a Bosu ball or step.
Butt Blaster Most gyms have a Glute Isolation machine. As a variation, you can also perform this exercise by sitting backwards in a 45 degree leg press machine and pressing the weight up with one leg.
Hip Abductor Machine This exericse will work the sides of your glutes and your hip abductors.
A great variation of the Hip Abductor is instead of sitting in the machine as the video above shows, kneel on top of the abductor pads backwards (with your butt facing away from the machine). With your body upright, press the abductor pads straight out. It is an amazing variation that will really isolate your glutes. Once you're comfortable with the form, you can push quite a bit of weight with this exercise. It may look very awkward, but let that keep you from doing it. It's a killer exercise.
Leg Curl Machine Butt Blaster Just don't let your foot slip off the pad :)
Cable Pull-Throughs
Elliptical machine If your gym has Elliptical cardio machines that offer an incline mode, use it. Experiment with the incline (either half or all the way up), turn the resistance up high, lean your body back so you're spine is completely upright and push through your heels. This is highly recommended and very low impact on your knees.
Stair Stepper This may require you to bend your knees too much, but you can give it a try at high resistance, spine straight up and pushing through your heels.
Other Exercises This video features a mix of glute exercises, some that would work great for your circumstances, and other you probably can't perform. But its worth a watch.
