if your comments are a positive contribution they may be welcome. in code review it depends on what your role there is or was. if it involves parts that you have contributed to (and thus have experience with) then it should be fine.

the org that I worked for last year

one of the goals of GSOC is to motivate you to continue to contribute after your GSOC project completes. so you should certainly be welcome to get involved. many GSOC participants also volunteer as mentors later.