Yes, some lectures are taught by GSIs. Next semester, a class I might take, Rhetoric 20, will be taught by a GSI. It should be noted that, when a GSI is teaching a lecture course, it is usually a GSI very late in the game. For instance, the GSI teaching Rhetoric 20 is going to be hired next year as a lecturer by the rhetoric department. Sometimes this isn't the case, but it usually is. Anyway, as the Poli Sci 2, i haven't heard many good things about the two profs that usually teach it, so maybe you're getting lucky!