Learn Italian: Gli specialisti non hanno capito niente. - The specialists didn't understand anything.

Words ending in -ista like specialista are the same for men and women in the singular (lo specialista, la specialista). But in the plural, they change: -isti is for a group of men or a mixed group, and -iste is for an all-female group. So specialisti here means male or mixed specialists.

Why do we use the article Gli instead of I for this plural word?

We use gli for masculine plural nouns that start with an s followed by another consonant (like the sp in specialisti). The singular form uses the article lo (lo specialista).

Why are there two negative words (non and niente)? Doesn't that make a positive?

Italian requires a double negative when a negative word like niente comes after the verb. You must put non before the verb. While it looks like it means they did not understand nothing, this is simply the standard Italian way to say they did not understand anything.

Since Gli specialisti is plural, why doesn't the past participle agree and become capiti?