Since your weak signal area is above the gateway, you might try turning the antenna to a horizontal position and see if that helps. An antenna in a vertical orientation radiates most of the signal in a horizontal pattern and very little vertically, so it might be worth a try.
You're kind of limited by your choice the Comcast gateway (this model is from Netgear). Netgear does sell a number of antenna upgrades and range extenders for some of their products, but I don't know if they are compatible with the gateway you have.
I do agree with the other poster, try different antenna orientations, they can make a HUGE difference in signal reception. If that doesn't work, try placing the router in a different location, away from the wall and higher up off the floor.
Another option is to run an etherent cable from the gateway to a WAP that you have closer to where your wireless system is.
Baring any success with that, I'd cancel the Comcast Home Networking service and go get a better router with extended range capabilites, or with the ability to use more sensitive antennas. or add on wireless bridges/range extenders.
I personally use a Linksys WRT54G with an extra WAP54G that provides both etherent and extened range wireless capabilities upstairs (the router is in the basement). The WAP is in repeater mode and simply extends the range of the router's signal and provides a local ethernet interface for wired devices that are too far away from the router to connect directly.