Learn French: Le sonar détecte un calmar géant dans l'obscurité. - The sonar detects a giant squid in the darkness.

Yes, both spellings are correct and widely used. Calmar is often used in scientific or formal contexts, while calamar is very common in everyday speech and when talking about food.

Géant describes size. Since size adjectives usually go before the noun in French, shouldn't it be un géant calmar?

You are right that common size adjectives like grand, gros, and petit go before the noun. However, géant is an exception to that rule. It acts like a standard descriptive adjective and follows the noun.

Is there a reliable way to remember the gender of l'obscurité?

Yes! Abstract nouns ending in -té that express a quality or state (like l'obscurité, la beauté, or la réalité) are almost always feminine. The few masculine exceptions are usually concrete nouns, such as le comité (the committee) or le côté (the side).

How would we apply this lesson's grammar to say 'Whatever giant squid the sonar detects...'?

You would use quelque as an adjective modifying the noun. Since calmar is singular, it is written without an 's': Quelque calmar géant que le sonar détecte... Keep in mind that détecte is in the subjunctive mood here, even though it looks exactly like the present indicative for regular -er verbs.