Just like horselovingguy said "would you wearing a shoe 4" longer/wider be comfortable for you?" This statement is such a good statement. If you know your horse needs a 5.5-6" gullet then that is what you need to be looking for. Having one that is too large is going to cause pain and injury to your horse. also to your question of would a half pad help, No it won't, trying to fill that much of a gap could easily just cause more problems and not help at all.
There is a ton of helpful places to find information on how to properly fit your saddle to your horse. But one big thing is if you're not sure really how to do that find a good saddle shop near you. Most good saddle shops will let you bring your horse in and help you find a properly fitting saddle with your horse there. If you can't do that though here is some helpful info on saddle fitting your horse.