What they all missed was that German already had degendering built right in, the diminutive (something dearly missing from the English language) : der Holzfäller -> das Holzfällerchen, der General -> das Generälchen, die Kanzlerin -> das Kanzlerchen, die Maid -> das Mädchen
The gender is often derived from synonyms or etymologically related words. I'm guessing the reason it's "Der Computer" for example is that "Computer" works similarly to e.g. "Trockner" (drier) or "Rechner" (calculator, in fact often the German synonym for "Computer") and these are all male (as generally words ending in -er tend to be).
OTOH it's "Das Steak" (maybe because "Das Fleisch" or "Das Kotlett"?) and "Das Shirt" (maybe because "Das Hemd"?). The fallback generally seems to be "das", i.e. neuter, not male.
An example of something with inconsistent use is "Der Laptop" vs "Das Laptop". Presumably this is because "Der Laptop" is derived from "Der Desktop", which in turn is likely simply a shorthand for "Der Desktop-Rechner", making it male. Even the Duden (the closest thing to a "standard" dictionary in Germany) lists it as "der; auch das Laptop" (i.e. "der" seems to be somewhat more common but both are in use).
Some words are kind of gender-fluid too. E.g. a word meaning "hall" was female and neuter in past centuries, and now is male. The word for "coffee" is male in formal/educated speech and neuter is less-educated but (to my despair) rapidly gaining legitimacy speech. Russian is fun :)
For things like the user, you can always assume male, but as with other such assumptions, some people may object to it. In some cases, there is a built-in assumption that male includes female when gender is unknown, but in others it just means male and that's it.
FTFY
Two can play this game. There is no sane answer.
Except for rules of thumbs like "Romance-sounding word ending in -a is probably female".
On a semi-related note: in the Rhineland gendered articles are also used colloquially when talking about people: "Der Mark", "Die Steffi" and so on. I was born into this but I'm unsure how I feel about it after finding out it's actually non-standard in the rest of Germany.