I'm sorry you missed your flight, but gate changes do happen, especially @hub cities which IAH is a hub for CO. Unfortunately gate change announcements are sometimes limited to the immediate area or concourse where the original departure gate is located and even if the announcement was made to the entire terminal it's possible that with all the noise in the airport you did not hear it. Gate changes are ALWAYS updated on the video display monitors throughout the airport and as a frequent traveler I always double check those monitors especially when I am enroute to the departure gate to make sure there wasn't a last-minute gate change. Since I mostly connect through Houston my boarding pass for the connecting flight was issued sometimes hours before the departure time so I never rely on the gate information printed on the BP.
Having been through IAH quite a bit, I am wondering when you approached the gate you thought was correct for your flight did you not notice the display? At every gate @IAH there is an electronic display which has the flight #, destination, departure time, boarding time, and any delay information. Those diplays are controlled by Continental Operations so when the gate for your flight was changed, so too the info. on the diplay would have been updated to reflect the next flight's info. departing out of that gate.
Did the agent you spoke to actually look at your boarding pass? If so, I'm guessing s/he probably mis-read the flight information or just checked to see if you had a seat assignment and nothing more. It's unfortunate that the agent wasn't more attentive, but ultimately it is your responsibility to be at the proper departure gate.
If you wish to pursue this further, you can call CO @800.WE.CARE2 (800.932.2732) or write:
Customer Care
Continental Airlines, Inc.
900 Grand Plaza Drive NHCCR
Houston, TX 77067-4323