Learn Swedish: Jag gissar att kön är knappt en kilometer lång. - I guess that the queue is just under a kilometer long.

Both mean that the queue isn't quite a kilometer long, but knappt (just under / barely) puts a bit more focus on the fact that it falls short of the amount. It is specifically used for measurements and numbers as the exact opposite of drygt (just over).

Doesn't kön mean "the gender" in Swedish?

It can, but not in this context! Swedish has two very similar nouns. ett kön means "a gender," and its definite form is könet. However, en kö means "a queue" or "a line," and its definite form is kön. Since the sentence uses kön, we know it refers to people waiting in line.

How should I pronounce the k in kön?

Because k is followed by ö, which is a "soft" vowel in Swedish, it softens into a sound similar to the English "sh" or "ch" (like the first sound in "shoe" or "choose"), depending on the regional dialect. It is not pronounced with a hard "k" sound.

Why does the sentence use lång instead of långt?