Hi,
I'm so glad that you have found a good exercise routine. I commend you for following your preferences and buying a machine that will allow you to work out in the privacy of your home.
I consulted Michelle Hering, an MS in exercise science candidate at George Washington University, for her assessment, and here's what she advises:
I would suggest that you go see a physician or physical therapist to make sure there isn't any damage that was unknown prior to beginning your exercise program (such as arthritis or a small tear etc). Also I am not 100% familiar with the gazelle but if you could increase the incline so that your toes weren't pointing in a downward motion this should take some pressure off of both ankle and knees. So the gazelle motion should be more of an uphill cross country skiiing.
If the pain is acute (sharp not there all of the time) this is a sure sign to seek medical help, if it is chronic (achy, all the time, wish you could just take your leg off kind of pain) then it might be an overuse... Give it a rest for a few days, ice it and take some advil to reduce swelling. When it feels better begin the exercise program again and if the pain reoccurs seek medical help.
Thanks for your question, and let me know how you're doing.
Tracy