Summary for those too lazy to read the below: G Major/Ionian, G Major Pentatonic, G Major Hexatonic, and G Minor Blues are all usuable (but careful with the Blues scale).

The progression is in G Major; depending on the sound you're trying to achieve, you could play G Major (a.k.a. Ionian for you mode lovers), G Major pentatonic w/ b3 and maybe #5 thrown in for spice (think about bending the A and D notes to hit the b3 and #5), or the G Minor Blues scale for a bluesey minor over major type sound. Experiment. I'd say the most likely and ear friendly approach I'd take if playing over a given song with that progression for me would be G Major Hexatonic. That's basically G Major Pentatonic with the added 9th of F#, so it's every note in G Major except for the 4th (C), which usually just clashes unless you play it over a C or Am chord, anyway.