The tour starts at the ticket office/gift shop where there are bathrooms available. The staff there were very helpful and friendly.
I'd suggest you park near the cemetery side of the parking lot when you arrive since you will likely want to walk around Cemetery Hill upon your return. Cemetery Hill, with an excellent view of Culp's Hill, is directly adjacent to the huge free parking lot .
Our tour from start to finish lasted 1 hour, 51 minutes. The tour lost an additional 10-15 minutes driving to and stopping at a coffee shop that offers a bathroom break midway through the tour. Actual Informational Touring Time was ~96 minutes.
Because there were other tour buses already parked at Little Round-Top when we arrived, we stopped at the far north end of the site rather than the main summit. Our guide shortened his presentation considerably at this stop, which while disappointing, was understandable. Luckily, hubby and I had driven to and walked around both Little Round-Top and Devil's Den the previous day. There is a spectacular castle-inspired monument at the summit of Little Round-Top with a staircase you can ascend to look over the entire battlefield. If your tour does not stop at the proper summit, I strongly suggest you go there on your own. It is well worth visiting and missing that site is missing a lot.
In summary, this tour provides an excellent overview and an efficient way to see and understand the scope of the battle of Gettysburg. Our guide was extremely knowledgeable, answered every question and concisely explained the timing and pivotal moments of the battle.