Ride throughout the summer, increasing your mileage until a metric century isn't a rarity. I also recommend taking one or two short trips (1-2 days) so you know what to expect in terms of the effort required to push and handle a bike with a load. Are you planning to camp and/or cook? If not, you really don't need to carry much at all.
I did the GAP a few years ago as part of a cross-PA tour in mid-September. Pittsburgh to Connellsville the first day. Connellsville to Rockwood the second day. Rockwood to Cumberland and then north to Bedford, PA the third day. Until you hit the divide without 23 miles left, you will have a net elevation gain, albeit a gradual one. Rockwood to the divide is a little steeper than much of the other mileage. From the divide to Cumberland is a piece of cake requiring much less effort as it's a good net elevation loss as rail-trails go. Wind may also be a factor for at least part of the trip. The first day of my trip the area was under the influence of a weather pattern producing winds out of the SE. There were some open sections where I could feel myself pedaling into the wind. You also need to train your mind. You will likely be going slower than you do during road rides. You need to accept it and not push yourself too hard. Despite more than 15K touring miles I still sometimes start out too hard. It's good way to wear yourself out early on. During my Black Hills tour back in June I did 61 miles and nearly 4,800' of climbing in the first day. I was on the road for only 7.5 hours, including two extended stops and numerous shorter ones. Day 2 I did 71 miles, 41 of which were on the rugged Mickelson Trail, with a total 5,000' of climbing. Pushing too hard the first day made Day 2 very hard. I was pretty tired by the start of day 3.
BTW...Mid-September was a nice time to ride the GPA. I left PGH on a Saturday morning. Not much traffic on the trail, and I camped alone at Connellsville. Only saw one other person who was travelling loaded. Sunday I encountered a greater number of day riders, but not enough to cause any congestion. Had Husky Haven Campground all to myself. Monday morning I saw two day riders heading in my direction and a family heading in the other directions. Days were warm but not unpleasantly so. Nights were cool, but not cold. I really liked the two places I stayed. The dollar store in Rockwood had a poor grocery selection, but there are a couple of places to get prepared food, including an inexpensive pub on the east end of own. Just bring ear plugs if you are a light sleeper. Even though they are across the river from the campground, the trains can be very loud as they blow their horns at the road crossings in town.