First off, a qualification: as this amounts to guessing what was in a particular person's mind when they coined a phrase, this answer is somewhat speculative.
Outside of cathedrals, gothic isn't a straightforward word at all.
Take typography: a fairly limited and, ultimately, a practical field. Here, gothic has probably been stretched to breaking point in terms of what it can encompass.
It can be used to refer to blackletter, a traditional European style often used in English to imply times past (think: ye olde Shoppe) and to evoke authority (for example, in newspaper mastheads). Despite its longstanding tradition in German-speaking countries, blackletter was condemned by the Nazis in 1941 ("Jewish letters," apparently), having been in very common use in Germany up until then, and even the usual body type in German books.
However, gothic is also used to refer to sans serif typefaces, which are geometric and unadorned in their reductionism, quite unlike ornate blackletter hands. These sans serif typefaces, in particular the more stylised geometric fonts, were favoured by advocates of the modernist Bauhaus movement. However, once again, this very different flavour of gothic type did not meet with National Socialist favour ('cultural Bolshevism' this time, apparently):
While both these styles are legitimately called gothic, as you can see, blackletter and sans serif are almost polar opposites, inasmuch as such a thing is possible in an aesthetic field.
Raygun gothic is geometric in its influences, and owes more to art deco than anything - not surprising as the two emerge contemporaneously. (The Chrysler Building was finished in 1930, two years after Buck Rogers and four years before Flash Gordon appeared on the scene.) Sans serif faces were also in vogue and rising in importance when art deco took over in the 1930s; as has been previously noted, these typefaces are also called gothic. In the 1930s and the 1950s sans serif faces and raygun gothic both saw prominence.
The same Wikipedia article you reference states that William Gibson, the science fiction author, was the originator of the term raygun gothic in a story called The Gernsback Continuum:
Cohen introduced us and explained that Dialta [a noted pop-art
historian] was the prime mover behind the latest Barris-Watford
project, an illustrated history of what she called "American
Streamlined Modern." Cohen called it "raygun Gothic."
(I've searched for comment by Gibson into his thinking here, or even authoritative comment - I can find nothing.)
To go back into typographical and graphic design history, the International Typographic Style came together in the 1920s, rose in the '30s and crystallised in form in the 1950s, with Univers and Helvetica being perhaps the two most notable typefaces of the time (and still now). Akzidenz Grotesk, Helvetica's granddaddy, is somewhat earlier, but the motivation behind its design is the same:
In 1896 the Berthold Type Foundry released its Akzidenz Grotesk
Typeface in an attempt to capture an objective style, and from that
point the International Typographic style evolved as a modernist graphic movement that sought to convey messages clearly and in a universally straightforward manner.3
(Emphasis in the quote is mine.)
Note also that in German, grotesk can mean both grotesque and sans serif, not unlike gothic in English - one need only think of a gargoyle to see the overlap. Indeed, the origin of gothic as an architectural term is as a sweeping insult applied by renaissance Italians to the grosser medieval styles of northern Europe.4
To return to the Gibson quote and raygun gothic, as a phrase, American Streamlined Modern evokes exactly the same spirit as the forward-looking sans serif grotesk, gothic faces of typography: American Streamlined Modern even sounds like a font name, and it would surely be a sans serif face.
I'd suspect that this is what Gibson had in mind in his use of the term, or perhaps it served as a hook to hang it on... along with a very large measure of what you yourself said - making it sound like an 'official' art movement. Much in the same way, there isn't much punk about steampunk or dieselpunk, and it doesn't particularly matter. (Incidentally, the alternative names rocketpunk5 and atompunk6 are also used for raygun gothic.)
More by the way than anything, in American Gothic there is a rather incongruous gothic window in the house behind on the upper floor - gothic in the proper medieval sense, here (or, perhaps more accurately, gothic revival). The juxtaposition of the light wood-framed house and the weighty pretension of the gothic window set in it amused Grant Wood, the artist, and he painted the house, along with some suitable residents for it in the foreground.8
In this instance, at least, gothic is used in the 'proper' limited architectural and art history sense, and not as a catch-all, as you suggest.