What year is the D-25? They were the entry level dreadnaught in the Guild lineup for years but they're known for excellent tone and they've held up exceptionally well.
The D-25's were largely made in Westerly, RI. The GAD's are made in China. The D-25's have a nitrocellulose finish, the GADS have a poly finish. The GAD is rosewood, and has excellent workmanship, but I still don't think it can hold a candle to the mahogany D-25 tonally. I've played many, many of both models, and the D-25 sounds way superior to the GAD-50.
If you can find a nice old D-25, snatch it up. Pre-about 1974 they had a flat back and most had a spruce top but some had mahogany. After that they had an arched laminated mahogany back with a spruce top, like mine. I bought mine brand new in '76, and it's still ringing like a bell, and it's held up through a thousand campfires and gigs.
I can't speak to the later model D-25's made after Fender bought out Guild because I've never played one, but anything built in Westerly is likely to be a nice guitar. The jury is still out on Corona, CA built Fender/Guilds, they have a "round" serial # sticker inside, Westerly built models have an "oval" serial# sticker inside. (Easy way to ID them)
Summary: (disclaimer - this is a biased opinion)
You can get a nice vintage D-25 for less than the cost of a GAD-50, and you'll be happier with it.