I also live in a townhouse in Ashburn. After some research online, we came up with the same dilemma with the steps up from our entry to our living room. The angle and width made it really complicated, so we decided to look into hiring a baby-proofing company, Safe Start Baby that I think is located in Round Hill. You can get an in-home assessment for about $100 and they will tell you the areas of concern in the rooms to be baby-proofed and what types of gates they would use.
We chose to have them install the gates (my husband works a lot and I'm not that handy with a drill!) and they did a great job. We had to have a couple of different types of gates in our house. They didn't drill into any of the banisters but instead used zip-ties; they did have to drill into the wall and into some trim, but those can be patched up easily. They are all super easy to remove as needed, as we discovered when we had a new refrigerator delivered.
When we hired Safe Start Baby, they didn't have an online store...but they do now! You could get the assessment and then buy the gates online and install them yourself. (Installation is not cheap!)
Hope this makes sense...if not, please email me!