I'll be totally honest, for me GB/FB is a number I don't usually look at. One of my favorite pitchers of all time,
Jair Sanchez had a GB/FB of 0 and was still very servicable. Now, keep in mind, this is in San Diego, where hit baseballs go to die.
Generally I'd say look for GB/FB velocity around 30-50 for starters and 50+ for relivers. For starting pitchers, they face so many batters and are in so many situations that striking a player out or keeping the ball on the ground for double plays tends to even itself out over the long run of a full season or multiple seasons.
But for relivers, especially ones that are brought in for high leverage situations, getting a strikeout, a double play or a infield fielders choice is far more important then getting a flyout that could advance the runner or score a run.
If I have the money available at the end of the season I will sign everyone from the draft I can starting from the cheapest unscouted guys and working my way up. There are sometimes a few gems that get unscouted by everyone that will end up being staples in your ML roster.
Tony Puljos was taken in the 19th round and wasn't a diamond in the rough. No one else saw him and I was lucky enough to sign him. You never know.