Alvin Plantinga’s notion of warrant (justification) is a form of externalism (reliablism).
[A] belief has warrant only if it is produced by cognitive faculties that are functioning properly in an appropriate environment. Plantinga’s notion of proper function, moreover, implies the existence of a design plan, and a belief’s having warrant requires that the segment of the design plan governing the production of the belief is aimed at truth. In addition, the design plan must be a good one in the sense that the objective probability of the belief’s being true (given that it’s produced in accordance with the design plan) must be high.
However, Plantinga does not avoid the Gettier problem. The Gettier problem challenges the notion that knowledge is a justified true belief. In short, the problem is about accidental knowledge ([K], if there is such a thing) and having a belief that is true while your reason for justification is false.