I have given it a try, but I usually run 2 walking tyrants as my only synapse, and the recylced gaunts seem to always be falling back at the end of the game.
Yup, this is certainly an issue, and one of the major strikes against. What I end up doing is setting up a "Zoanthrope Highway" with my 3x Zoanthropes, so that when the Guants come on, they've got Synapse all the way to the other side of the table, or at least where I need them to go. This doesn't always work, even with 3x Zoanthropes, so I can see how 2x Tyrants would be a struggle.
The good news, though, is that even if they come on oot of Synapse, they're either going to do what you want (unlikely), or move quite quickly up the field towards your Synapse.
But I keep them small to make sure they get wiped out so that I can shoot and assault my opponent's squad next turn.
Right, that's exactly what I'm suggesting, and I find that 12-14 does this well. All they need to do is keep one guy in the fight until his turn, and the unit they're on is locked up.
A major rule in
40K is "win combats during his assault phase." But second to that is "lose combats during his assault phase." Because even when the Gaunts lose, they'll be right back, and there's 2 Dakkafexes waiting to smoke the squad that just killed them.
Or, I guess not smoke, so much as "cover in bitey maggots."
Bottom line, you have to pick a brood size, and learn how it will do against different units in different situations. That's where the Mathhammer helps a lot. For example, you know you can't run them into Khorne Raptors, because the Raptors will just laugh, take the extra
D6 move, and then assault your Fex. Let the Raptors have them, then use your next turn to deal with the Raptors. 126 points of Gaunts die, then your whole army can move in and finish of 250 points of Raptors with no problem.
Nothing better than a speedbump unit that comes back to do it again.
The big lesson of Gaunts (and this isn't news to some people in this thread) is that you have to almost hate your Gaunts. That's why I say "Gaunts suck." It's the most important rule of Gaunts, you can't get attached to them, you can't start planning for them to accomplish anything, you have to know that they're stupid little clods who exist to hinder and irritate your opponent. Like Grots, but with more menacing fluff.