Guerrillera is guerrilla rank 2 in Libertad. Reaching it opens the next set of weapons and gear for purchase from Juan and his Arms Dealers.
Your guerrilla rank rises as you earn XP across Yara. Each new rank opens a new batch of weapons and gear for purchase, and some items say right in their unlock text which rank you need. The game shows either the feminine or masculine form of each title.
Keep clearing checkpoints and capturing FND bases. Captured bases also add Arms Dealers where you can spend your pesos on newly unlocked items.