Cheapest might depend on what you want to when you get there. Business meeting? Sight-seeing? Pirates games?
I think any sight-seeing at Gettysburg would require a car or taking a bus tour from Philly. If you have meeting at a specific place in/near Gettysburg, there are shuttles you can take from Harrisburg; the airport website lists them.
Harrisburg: you can both get there and get around with public transit. There might be a sight-seeing bus which includes both cities.
Don't have any info on Pittsburg.