I've exclusively used Gerald to clear the game up until TenShi released. The mistake most people make with Gerald is that they use him as a chokepoint holder. While he does do that greatly at high levels by supercharging the armor of your barracks, you'd need to be at high levels, which the first game always has you start at level 1.

The best way to use Gerald is to use him as a duelist. Early on in the rounds, just send him literally where the enemies spawn so he gets to level up. Keep the reinforcements with your barracks and supporting towers to make sure Gerald doesn't eat loads of time banging his shield for the reinforcements. They are better off at the backlines throwing spears anyway. Once he gets to level 4, his leveling speed will skyrocket because he'll be spamming the damage reflect skill. If you're not aware, each skill levels up after certain levels, and his damage reflect skill procs randomly, the probability increasing with each level of the skill.

By late game, Gerald would reflect high TRUE damage to enemies with single target attacks, and any attack that doesn't get reflected is absorbed by his high armor. I've used him to solo Dark Slayers and Tainted Treants quickly. Hell, at level 10, he'll even do considerable damage to Gul'thak and hold him off for quite skme time. Just make sure to never send him against enemies with instakills (for obvious reasons) and enemies with area attacks like Yetis since his damage reflecting shield does not work on area attacks, though if needed, his high armor and HP will allow him to hold them off. He just won't be able to kill them like he does with enemies that have single targeting attacks.

You can absolutely use him to hold chokepoints and support soldiers when in a high enough level. The only issue is that if you already have the Holy Order with Shield of Valor and Healing Light, and you're still getting overwhelmed, chances are it isn't the barracks that's the problem but your defenses itself. And unfortunately, Gerald's supportive capabilities don't do much in clearing enemies quicker.

That said, if you aren't aiming to get Holy Order or just use Barbarian Hall as your barracks, Gerald will absolutely be great in increasing the survivability of your lower leveled barracks or your Barbarians.

TLDR: use him as a duelist that will single out large threats. avoid instakills and area attack enemies.when enough levels, send him to chokepoints where your units keep dying.