Bones is right, you can stop them at the bridge south of Thapsus. I just started a Carthaginian campaign, and within the first six turns I was able to get a full stack at the bridge to meet the Gorgons. I did it by sending a ship to Palma and bringing back the General and Balearic Slingers there (hiring a merc slinger before leaving), bringing a good piece of the Sicilian garrison back to Africa (after using them to take Syracuse), hiring local mercs (you can get Hoplites at the bridge), and of course raising my own troops.
I just destroyed the first Gorgon stack at the bridge, and will take on the second once I get done with the laundry and shopping. I was taking screenshots, but even though I thought I turned Fraps on, apparently I was wrong... The Sacred Band did not take a single loss in the battle, and overall I lost only 90 soldiers compared to no Gorgon survivors.
This is on Very Hard/Very Hard of course.
I just had round two against the Gorgons. This time Fraps was working.
Here they come. They have me outclassed in missiles, but my army is actually stronger in melee with my Generals, Sacred Band, and Mercenary Hoplites.
And so it begins. There are two crossings. I put my main army at the center, and place my cavalry at the side.
I wait at the center with my Sacreds and Hoplites in an open V, Mercenary Peltasts are behind the Hoplites on the left (this will let them throw their javelins at the unshielded right sides of the Gorgons). My Faction Leader and Elephants are behind the infantry to lend moral support and scare the crap out of the Amazons respectively. I leave my five Iberian Infantry units far to the rear. I will not be using them at all in this battle.
After a long drawn out archery duel which cost many of my slingers, the Gorgons finally start coming across. Since I see they are now committed to attacking in the center, I send my cavalry across the second ford.
The Gorgons are marching into a nightmare.
Now my cavalry are across and sweeping down upon the Gorgon archers on the riverbank.
In the center the Gorgons are hitting me hard, but not denting my heavies.
There comes a certain pivotal moment in every battle where the outcome is decided. After you play RTW enough you will be able to intuitively recognize when it is drawing near. This is when you want to throw everything you have in to swing that pivot in your favour. I felt that moment, and threw my Faction Leader and Elephants directly into the onrushing Gorgons.
They are sent packing.
Only a few of their cavalry were able to escape.
Scratch a second Gorgon stack.
I will not be bothered by them again.
The rest of the horde sacked Triton while I was busy destroying those two stacks.
Next turn they moved west. They want no part of Carthage.
I pursue. Time to go horde hunting.