Learn French: Le bateau continue malgré les vagues gigantesques. - The boat continues despite the gigantic waves.

No, malgré is a direct preposition. Just like the English word "despite," it doesn't use "of" (de). You just put the noun directly after it, which is why we say malgré les vagues (despite the waves) rather than adding a de.

Could I use bien que instead of malgré in this sentence to mean the same thing?

No, because bien que (although or even though) must be followed by a subject and a conjugated verb. Since les vagues gigantesques is just a noun phrase with no verb, you must use a preposition like malgré. To use bien que, you would need to change the sentence structure entirely, such as: Le bateau continue bien que les vagues soient gigantesques (The boat continues even though the waves are gigantic).

Why does gigantesques come after vagues instead of before it?

In French, most adjectives go after the noun they describe. The main exceptions are short, common adjectives relating to Beauty, Age, Number, Goodness, and Size (the BANGS rule) like petit or grand. Even though gigantesques describes size, it is a long, multi-syllable adjective, so it follows the standard rule and is placed after the noun.

Is gigantesques the masculine or feminine plural form here?