GA and SRO stand for General Admission and Standing Room Only, respectively. These are terms used for sections and rows that do not have assigned seats and where you sit or stand is on a first-come first-served basis.
Most of the time, if you purchase tickets such as "Pit GA", you are required to stand the entire event and may be fairly close to the other people in your section. If you have "Lawn GA" tickets, there typically is room to sit on the ground if you choose to view the show from further back.
Even though these sections do not have assigned seats, the tickets may have numbers or letters in the section and row fields. This is for the venue to keep track of inventory and does not affect where you may sit or stand.