I must admit to having not built that particular kit...I am, however, working on my 3rd biplane of some sort, and was, not so long ago, a new kit builder myself. So, this post is more "general building tips" than it is specific to that kit. These are, essentially, a few things I've learned along the way, and while they by no means represent a large pile of knowledge, they might help ya with your first build.

1) Glue has weight. It is, in almost every case, nothing short of surprising how LITTLE glue of various types is needed to effectively bond two surfaces. Obviously, how much is needed, and how long it should be clamped/pressed/held together depends a great deal on the type of glue and surfaces being bonded. However, it is WELL worth your time, imo, to spend a couple of days experiementing with various types of glues, joints, and materials to find out just how little you can get away with. It may SEEM like a trivial detail, but just a couple extra ounces of glue per wing panel makes for HALF A POUND of extra weight on a bipe. That's significant on ANY plane, and HUGELY so on a small 40-90 sized aircraft.

2) The secret to a bipe build is to take the time to be EXTREMELY precise in everything you do, particularly the wings. Obviously, this is true of any aircraft, but I've found it to be doubly so for bipes. Given their very nature (bipes are DRAGGY airplanes) their slow speed performance is frequently "touchier" than most planes to begin with, and ANY flaw (improper lateral balance, warped wings, incorrect incidence angles, etc) tends to be exposed during that phase of flight.

3) Sand everything, sand it again, sand it one more time, and then sand it. Smooth, clean, precise fits and surfaces not only increase the strength of your model, but present a much cleaner appearance when covering begins. That little "lip" where those two pieces meet may appear insignificant now, but shrink some covering over it, and watch it collect dirt and grime, and suddenly become a glaring "flaw" in the finish. Remember also that when two pieces join, a perfect flush fit between the two pieces will form a much stronger joint.

As a side tidbit, I'm presuming from your original post that you've not flown a bipe before. No worries, the GP Ultimate is a nice flying airplane, and shouldn't throw many surprises at you...I agree with others above, with a little care and caution, you should be able to fly it just fine. There are, however, a couple of bits of "bipe advice" I'll pass along:

As I said above...draggy critters, bipes. Yes, they DO get "more lift" (a misnomer if ever there was one, but we'll leave that go for now) than a single wing, but that extra wing, along with the struts, cables, etc that attach it, all add up to DRAG...and lots of it. As a result, you'll likely notice a bipe doing a few things your Extra and SkyRaider don't do.

First, as Colin said, when landing, KEEP THE POWER ON until you're dead certain you have the strip...IMO, it is NOT true that bipes "fly worse" at slow speeds than monoplanes, but their "line" between flying and falling like a rock is MUCH thinner, and happens MUCH more quickly. Your extra will start wallowing around a bit, getting mushy controls, that sort of thing, for quite a while before it stalls. Most bipes, on the other hand, will go from "flying" to "rock" in a nanosecond, and the warning signs, while they ARE present, are notably different, and difficult to pick up on at first. So, absolutely, when landing keep the speed up where you KNOW it's flying...you can always go around.

Also, be prepared for the idea that, because of the extra mass in the "same space" as an equivalent sized monoplane, a bipe tends to "overdo" everything. Aileron and snap rolls, for example, tend to KEEP ROTATING for a while after you center the sticks....in fact, I have one bipe (a giant aeromaster) that will snap at LEAST 90 degrees past the point where I command the snap to stop. Again, not a problem, but it might catch you by surprise the first time or two.

Finally, I really just have to say this...

Get the Ultimate, have a blast building it, and then go fly it...you'll learn why "real planes have more than one wing".

Enjoy, and good luck. Hope I haven't rambled too much.