Bold statement time: if you deckbuild for him, Gimli's not an offensive hero, but a defensive one.
Let me explain; due to his ability, you want to stack damage on him, and, due to said ability, it's actually better to give him boosts than boosts via attachments. So, you use the painfully obvious combo every Core Set player's done and put Citadel Plate on him. But it's outside of the Core Set that I'd argue Gimli reaches his greatest level of power, if you stack him with (I will admit, typically expensive) boosts, and some healing.
Everyone knows how defending works; you deal an attacking enemy a shadow card, exhaust a character to declare them as a defender, then look at the shadow card and resolve the attack. But Gimli with Citadel Plate dramatically changes this, especially once healing's in play. "Oh, the Temple Guardian's hitting 6 ? Meh, undefended on Gimli anyway." Simply put, Gimli plus Citadel Plate (and especially the second one, if you can find it) dramatically changes the game. You go from blocking with Defender of Rammas to just absorbing an attack undefended, even from the Host of Angmar. 's healing and even Dark Knowledge make Gimli even more insane. You'll be taking multiple attacks without exhausting, the shadow scrying of Dark Knowledge or Silver Lamp allowing you to see nasty shadows so you can respond properly. Gimli plus boosts plus healing I'd argue is even more absurd than Beregond with a Gondorian Shield, Raven-winged Helm, and plenty of Honour Guards for backup. Sure, Beregond can defend for infinity against an enemy-but only one. Gimli can eat multiple attacks, even from high enemies if you have healing, and still be ready to counterattack.
And can he counterattack; once you get some damage stacked on him, Gimli effectively has X , where X is the total bulk of the bulkiest enemy in the encounter deck. For example, with 2 Citadel Plates and maxed out damage, Gimli can one-shot any enemy with less than 14 total bulk. Not only does that mean that the Eastern Crows definitely aren't getting up again, it means that you effectively have an ideal boss-killer; not only one who can most likely one-shot them, but one who can defend against them, too. But even damage up to his maximum printed still gives him 6 , the typical magic number for destroying enemies.
Another nice thing about Gimli defensively is how he dodges restrictions on declaring defenders. Witch-king's anti-chump/Beregond "no declaring characters with less than 2 as defenders against the Witch-king" getting you down? Just take it undefended on Gimli. Uruk of Mordor's inability to be defended against by Defender of Rammas or Snowbourn Scout getting old? Defenderless Gimli. Druadan Warrior attacking when you're out of resources? Just dump it on Gimli. Sure, that's minor, but it's minor like Power of Orthanc; not that useful in most quests, but downright absurd in the few it has use in. And all this is ignoring Vigilant Guard.
Overall, Gimli may be an oldie, but he's still a goodie; sure, he requires a lot of support, but if you give him that support, if you build your deck around him, you'll find yourself blinking at the sheer amount of defenderless attacks you're tanking. Word of warning, though; many are the times you'll scream at an enemy, "You couldn't get +1 , could you?" Gimli may require a lot of support, but he's well worth the deck space and resources it takes.
(Also, Grappling Hook is way too fun with Gimli; it's fun sending a character on the quest with more than the Outlands player.)