Remember two things about the Gatewood ...
No bug protection
No Floor so ... you have to carry a groundsheet
Remember two things about a Bivy
In addition to limited storm protection, you get 5 to 10 degrees of warmth out of a bivy.
Can act as bug protection and a ground cloth.
You can get by with a Headnet and a Polycrow ground sheet for a couple of oz, but you'll be shopping for a bug net after you use it a few times. (The first time I ever saw a Black Widow spider was when she crawled over my hiking shoe when I was sitting outside my tent in the Sam Houston National forest just north of Houston (I would have sworn that she was as big as a hamster at the time), I've been real big on bug protection when sleeping ever since).
You'll be warmer in a GW cape than out under the open, most tents add 5 to 10 degrees on their own.
A Titanium goat bivy, at 6 oz, can replace the need for the bug net and the ground sheet (if you're not using Nano), plus add another 5 to 10 degrees to your bag. If you haven't bought a good down bag yet, this system can stretch a Western Mountaineering 16 oz. 35 degree Highlight to almost a 20 degree bag (at around $250)(I sleep warm, so my 32 degree Montbell number 3 will go down to the teens with my insulated jacket on and a decent pad under me, in my GW cape and my Bivy).
The point of all this is that I'd suggest that you take a look at your insulation, shelter, Bag, Pad, Ground Sheet, and Bivy or no Bivy as part of a total system.
I bought a Coleman Canyon 32 degree bag (more like a 45 degree bag) at 2lbs 10 oz, made a homemade bivy out of $1 a yard nylon and some $5 Silnylon 2nds on my mom's old sewing machine, a campmor 200 weight fleece jacket (on sale for $25) and then bought the Gatewood cape. I had bought some less intelligent purchases along the way as well, but this was the strategy I settled on. I then went to a Montbell # 3 bag for $260 (I need a 67 inch girth bag for my wide shoulders) at 23 oz, found a Patagonia micropuff pullover on summer clearance, and am working on a new Bivy now. (the fleece and colman bag, although heavy, act to take my system down to the single digits if I need it now)
(I have a $70 Jetboil for when I take my wife or son out, but 95% of the time I use a great little Alcohol stove and a Ti cup, I have a TarpTent for when I take my son with me, but I use the Gatewood/Bivy for solo)
The next piece of my system will be a Bivy Liner quilt and a light Climashield Blanket. The Blanket will be used for warm Texas spring hiking and the Bivy and Bivy Liner will be used with my bag to extend my system down into the single digits for only an additional 8 oz or so, over what I normally carry. (3/4 of an inch of loft plus 1 and 3/4 inches of loft (2.5 inches of loft) plus the effect of the pullover, bivy and the tent, with a fleece balaclava)
So ... if you plan ahead, add up the prices for each into a grand total, and then make planned purchases when you catch things on sale, as you save up some money. But also remember that a good bag in the 30 degree range will cost less than a good 20 degree bag.
Ryan Jordan has a great article for premium members on this site on Sub Ultralight hiking .... which, even if your not looking at jumping into a 5 lb baseweight will give you a LOT of things to think about. It's worth the price of the subscription alone I think.
One last point. GossamerGear has Glen's Ultralight makeover DVD on sale for $5. He helps a (pretty good looking at that) young lady move from her normal 35 lb pack load to 8 lbs with strategic gear replacement. It's well worth the viewing IMO and cheap at $5 for the info you can gleen from it).
Good Luck and Happy Trails