The Wikipedia article on gussets starts with the basic definition that "In sewing, a gusset is a triangular or square piece of fabric inserted into a seam to add breadth or reduce stress from tight-fitting clothing." While the Wikipedia article on gores states that a gore is "a segment of a three-dimensional shape fabricated from a two-dimensional material. The term was originally used to describe triangular shapes, but is now extended to any shape that can be used to create the third dimension." It should be noted that the gore entry is not specific to sewing; it's actually listed as a cartography article, although it also mentions parachutes, hot-air balloons, and round corners in duct-work.
These definitions lend themselves to the idea that a gore is used for the purpose of shaping while a gusset is added for the purpose of movement. I'm not sure why this would be different in corsetry, besides the obvious need to distinguish between a gore placed between to panels and a gore placed in a slit in a panel. However, given the close relationship between corsetry and costuming, I would think that corsetry-specific definitions for "gusset" and "gore" would actually get more confusing, since the use of gussets is so widespread in historical clothing.
Related side note:
There is also a brand of jeans called "Diamond Gusset" that uses (you guessed it) a diamond-shaped gusset in the crotch. While this would again lend itself to the theory that gussets are about allowing for freedom of motion, I'm seeing some problems here. For one, gussets aren't exactly a new idea, and these are being called "the original" gusset jean. Beyond that, they're referring to this gusset in the crotch of their jeans as a "our trademark gusset." Stop right there. Trademark gusset? At the top of their what is a gusset? page is the question "Why didn't someone think of this sooner?" Since when are crotch gussets a new idea? Or is the newness simply in applying it to jeans? Are jeans considered a different item of clothing than pants? I'm fairly certain that the term "jeans" refers to denim pants, denim being a specific type of twill. The variety of specialized fabric weave types is certainly fairly new, but can we call something new when it's an old idea, just a new fabric/pattern combination? Could there still be "original" crotch-gusseted khakis? But let's move on to the real problem with this statement about a "trademark gusset." 15 U.S.C. § 1052 states that registration of a trademark shall be refused if: "(e) Consists of a mark which (1) when used on or in connection with the goods of the applicant is merely descriptive or deceptively misdescriptive of them, (2) when used on or in connection with the goods of the applicant is primarily geographically descriptive of them, except as indications of regional origin may be registrable under section 1054 of this title, (3) when used on or in connection with the goods of the applicant is primarily geographically deceptively misdescriptive of them, (4) is primarily merely a surname, or (5) comprises any matter that, as a whole, is functional."
If we were to assume, for the sake of argument, that a gusset could possibly be considered a trademark to begin with (which I highly doubt) part 5 is clearly problematic. Their own site states rather clearly that the gusset is functional; it's not a mark of distinction, it's a method of providing superior characteristics to their jeans, so the likelihood of this diamond-shaped gusset being a trademark strikes me as about zilch. There's good reason for this limitation on trademarks-- functionality is legally governed by patent law. If someone could trademark a way of making something, that trademark would be so ingrained by the time that patent expired that it would be impossible for anybody else to use it, despite the expired patent putting it in the public domain. Not only would covering functionality under trademark law be redundant, it would effectively remove all time limits on patents.
Of course, I'm being pedantic, and given that I see no legal statements or legally representative trademark symbols being used, after having written all this, it's now occurring to me that they're probably using the phrase "our trademark _______" in the colloquial sense of "something about us that differentiates us from the majority of similar products on the market," rather than in a technical, legal sense. Furthermore, those Diamond Gusset jeans are both reasonably priced ($50 jeans, $120 protective motorcycle pants) and made in America-- not only "Made in America" but completely made in America, down to the American-grown cotton! Their customer service information page states that they have a 90-day guarantee on materials and workmanship, and a "fair-play" statement that if something goes on sale within five days of having ordered it, you can give them a call and they'll adjust your order to match the sale price. So now I'm somewhat regretting having objected to their non-technical use of the word "trademark" but hey, I linked to them four times! And I've never claimed not to be a pedantic geek.
Erm, I had more specific ramblings about gores, along with pictures, but I'll save that for another post, and leave this one as a post about definitions. Suffice it to say I'm now satisfied with the distinction that gores are usually-triangular pieces used for shaping and gussets are usually-four-sided pieces used for providing an increased range of motion. That does make it difficult to distinguish between set-in-panel gores and set-between-panel gores. Gore type 1 and gore type 2? Gore A and gore B? Gore B and gore I?