Gbetos counter their counters.

I saw a SotL video on UU counters and it's interesting he suggests archers, Hand Cannoneers, heavy cavalry and Onagers or Seige Onagers as a counter unit to Gbetos.

In my post-imperial tests, that was not what happened.

9 Gbeto vs 6 Paladins can go either way assuming that micro is available on both sides Against 6 Cavaliers it tends to be a slaughter in favor of the Gbetos if any micro at all is involved. Conclusion: heavy cavalry are at best equal to Gbetos or a good way to force the Malian player to focus on winning that fight. If your heavy cav aren't fully upgraded, don't bother.

7 Gbeto vs 9 Arbs is a losing battle for the Gbeto. The Arbs have a significant range advantage and that seems to help tip the balance in their favor. So as long as the important thing is overall resources and not gold at late game market prices, arbs are an effective counter. Crossbows, however, are not. The Gbetos consistently survive with one unit left. Both lose at equal numbers which means gold costs favor the Gbeto.

Seige Onagers: These don't counter Gbeto. Not in anything resembling balanced numbers. 3 vs 9 or 3 vs 10 the Gbeto win, which shouldn't be surprising since they do 15x the damage of archers and have more health and move faster. Onagers are even less safe. Gbeto counter Mangonel line units, with even small numbers of them being able to focus down these incredibly costly units in one shot.

Hand Cannoneers: Gbetos actually do surprisingly well here. Hand Cannoneers can't aim very well and shoot pretty slowly. 19 Hand Cannoneers vs 20 Gbeto is a pretty even fight and could go either way. 10 Gbeto consistently beat 9 Hand Cannoneers but 10 Hand Cannoneers consistently beat 10 Gbeto.

Perhaps surprisingly, one of the easiest counters is one he never mentioned. Hussars or even Lightcav can actually surround Gbeto in the time it takes them to attack, and are a trash unit that costs less overall resources than a Gbeto. 9 Gbeto vs 10 Hussars is unwinnable for the Gbeto unless they can fight in a narrow chokepoint. The Gbeto can focus down a few of them but end up being attacked from multiple sides and rendered unable to kite around like they can with heavy cavalry.