Since you are leaving on May 1st and posted the message on May 1st I can only assume you have already made the trip, but just in case you haven't.....
I have traveled it many times to Decatur, but never all the way to Gainesville. The road between Granbury to Weatherford to Decatur is a two lane highway with good road surface. The only navigation issues you may have are going around the courthouse in Weatherford!
Best way from Lampass to 51 would be to take 281 to Hico, take 220 out of Hico to Glen Rose, from Glen Rose take 56 to the 56/51 split, stay right on 51. When you get about 1/2 mile from Granbury, take Holmes (left turn) to 377. At 377 turn back south (left) and go to the 567 Loop around Granbury. Follow the Loop to Hwy 51 and take a left onto 51 and follow that all the way to Weatherford, around the square then on north to Decatur!
When you are about 1.5 miles from Granbury and passing Peak Road, look off to your right at a 200 foot cell tower. You will be passing our property at that point as that is our Cell Tower along with home to my Amateur Radio Repeaters :-)