Hello,
The beauty of the thing is that you can play a G minor pentatonic scale all along, it would sound awesome.
Then a lot of possibilities are available like the ones Ed talked about.
I would suggest you to get a hand on one (or all) of the Robben Ford video lessons, it will definitively help a lot.
To sum up the scale used by Robben Ford I would say:
1. Straight Pentatonic minor scale
2. Pentatonic blues scale (with the flatted fifth)
3. Minor Pentatonic with the major sixth (without the minor seventh), in G it would be G Bb C D E G
4. Major Pentatonic
5. The previous one would lead you to the Mixolydian mode, G mixolydian mode on the I chord and C mixo on the IV chord
6. The Superlocrian sounds good on the V chord
7. The diminished scale on any chord (for example G diminished alterning half step and whole step works on both the I and IV, but you'll have to change for D diminished scale on the V chord). The way Robben Ford uses it is generally to go from the I chord to the IV chord, i.e on the fourth bar of the progression.
Master these scales, put some bends and hammer off/pull off and you would sound like a great blues guitarist
One of the most important thing is to come up with your very own licks and patterns, put your personality into it.
Keep in mind that you can, maybe should, blend these scales, combine them. For instance, playing both forms of the third. Historically the third in a blues context is one of the blue notes, it's an african legacy where the third is neither major nor minor. Theorists refer to it as a "neutral" third or a "flexible pitch area". If you listen carefully to any blues guitarist, you may notice that they always put a slighly bend when playing the third of a minor pentatonic, and so does a singer. That's our "flexible pitch area".
I hope this helps,
Have a great time practicing
Cheers from France