I have not thrown a G-line FD, but I have read people on here say that they are far less stable than the S-lines. I have only thrown S and C in the FD, and the S is more mellow...don't go with C at this point for what you want (still a great disc). Perhaps the G-FD is what you want, but I haven't tested it so I can't say. The S-FD though is very close to a speed 7ish Mako3 flight in my opinion, while understanding that the speed 7 characteristics do change a disc. When you go up in speed, they will be a bit more nose angle sensitive, and as well there will be more low speed stability than a slower disc, no matter what.
The Leopard3 can be more overstable than people anticipate. But, if you have the power to make one turn then it will hold the turn. I remember the Gstar Leo3 being a good Leo type of flight, whereas a fresher star or champ may be more overstable than some people anticipate. I have not thrown a ton of these, just going off of a few generalities I've seen.
This is tricky, because I remember when I started out I bought a Pro Leopard from recommendations here as an understable disc. It was a laser beam for me, like a -0.5/1.5 type of stability when I threw it 275-300'. Eventually it became understable, somewhat from wear and somewhat from me getting better...and I lost it because it flipped too far right. So these discs will become less stable with time and experience. Also I now throw fairways in the 350-375' area so I am trying to keep in mind how these discs handle lower distances, but also powered up so they are useful for a wide range of skill. If I threw a Pro Leo at power now, I'm sure it would be a turnover/understable driver, but that doesn't change the fact that ~300' it's a laser.
As far as Gstar, it makes discs less overstable than Star and they do skip less. As well they don't kick quite as badly off of trees. In normal situations the less skip is often a good thing for placement shots, although some discs the skip is a good tool to have. For a straight fairway driver I wouldn't really plan on it for skip shots anyways, so I would buy the plastic that has the stability you want regardless of the moderate difference they have on impact. If you hit the ground early it won't slide as far on some terrain, but I'd count that as missing my intended line anyways.
The FD and Leo are very similar discs, I'd say that overall the FD is a little more neutral, and a bit faster/longer. They are both like a fairway driver of the Mako3...straight but with a bit of wiggle and they will have some ending fade when thrown under 300'.
If you want a fairway driver that turns to the right for either a turnover shot or "full flight" type of shot for under 300', you will need to look at another mold such as an Underworld perhaps. Also you could throw something like a DX Leopard (they are super fun) for a cheap way to figure out how a driver works for you and what you want out of it...as long as you don't play rocky courses that would chew up the low end plastic immediately. However, when you gain significant power these discs are farther on the understable end and would then just be useful for rollers/trick shots, etc. most likely. This is fine though...discs are not that expensive so getting the right one as a learning tool is useful too.
