I tow a 29', 7K lb TT with a 6.8l V10 F250 that has about the same power/torque as the 2015 F150 with the 3.5l engine. Steepest grade so far has been around 7%. Zero issues going up or downhill. Can run 60-65 mph uphill with power to spare but can see the needle on the gas gauge going down fast. Unless I need the speed for some reason, I slow down to save gas. Tow/haul works great and does it's thing.
I can drive down the steeper hills and have no problems. With our F250, we're well under the GCWR. Curvy downhill stretches are not problem either, but we have bilsteins on the truck and shocks on the trailer which helps a lot.
On downhill stretches, I would expect the bigger brakes on an F250 to make quite a difference. AFAIK, the 2015 F150 brakes are not upgraded despite higher payload and towing capacity? Again, the T/H works well downhill too. I also know that our truck and trailer brakes are in good working order. The only concern I do have downhill is the traffic and the drivers and I try to leave plenty of clearance ahead but isn't easy sometimes.
Hills? Bring them on, I love them. If you live on the west coast, you get used to them since they're everywhere. The twistier the better too. :)
Google says the grade on the IKE Gauntlet is under 2%. Is that right? If so, 2% is nothing.