Pros
I had a really great experience with GPI US, primarily due to how relaxed, organized, and flexible the facilitators were. The students were incredibly fun to work with, and very respectful to me and the other AGLs. The pay is fantastic for this position, at $28/hr when I was hired. You're basically paid to help students practice thinking and speaking in English by asking them lots of questions.
Cons
Can be hard to maintain relationships with coworkers due to the fast-paced nature of the program. Each program only lasts about a week, and you only are working a few hours each day. There is no overtime, so the hours you are assigned are the hours you work, and some weeks are much lighter than others. Could be difficult if you are someone with mobility issues due to the fact that you are expected to accompany the students around the city some days.