During the story of Fullmetal Alchemist: Brotherhood, Father carries out his ambitious scheme with the aid of seven homunculi "children," and they all have a role to play. But not all of them are meant to be classic shonen villains in combat. Some are more like tools or support units, like Gluttony, which makes him appear deceptively "weak" as far as enemies go.
Father's plan involves more than fighting the heroes. He needs to create a nation-wide transmutation circle, orchestrate many bloody battles across Amestris, assassinate people who know too much, track down his special sacrifices, and more. Gluttony isn't a fighter; he's a multi-purpose minion who backs up his fellow homunculi with his unique abilities. Just don't ask him to throw a punch.
Gluttony Can Track Enemies Down... But That's It
When Gluttony first appeared, he looked like an oafish villain with a plain face and a thick, lumbering body. He contrasted sharply with his squad leader, Lust, and the shapeshifting Envy. Together, though, they could accomplish any mission by pooling their talents, and Gluttony had a role to play. In that squad, it was Lust who gave orders and pursued leads, and she was also a skilled assassin who could kill anyone with her Ultimate Spear. Envy could gather intel with his shapeshifting, and Gluttony was in charge of using his nose to track down the target.
Lust can pursue leads on paper, but once the squad is out in the field, it's down to Gluttony to actually sniff them out with his strong nose, and then let Lust and Envy take care of the rest. He can identify any scent and track it anywhere, and even Wrath/King Bradley relied heavily on this ability. During a battle with the Xingese people, Wrath needed Gluttony to track down Fu, Lan Fan and Ling Yao, then engage them in battle. The Xingese agents quickly realized that between Wrath's eye and Gluttony's nose, there was no way to escape or take the homunculi by surprise.
What is more, Gluttony's strong nose can be borrowed by others, mainly Pride. During a battle in a dark forest against Edward's group, Pride realized that he couldn't easily track his enemies, while his partner Gluttony could but was being overwhelmed by the Xingese warriors. So, Pride simply ate Gluttony to obtain his sensory abilities and used that strong nose to chase down the unmistakable scent of his enemies. That turned the tables on Ed's group at once, even if Gluttony's powers came with a ravenous appetite.
The All-Devouring Gluttony
Gluttony is a poor fighter because he was designed not only to track but to dispose of evidence to keep Father's plans a secret. At least, that's how it turned out. When fighting his enemies, Gluttony can't use any advanced martial arts or ranged attacks; he simply tries to eat what's in front of him. Early in the series, Lust's squad dispatched Father Cornello, and Gluttony was tasked with eating him to erase all evidence of his death (and Envy assumed his form to maintain his role in the town).
Whatsmore, Gluttony was allowed to eat Ling Yao and other troublesome parties when he and Envy faced Ed's group. Even Roy Mustang (and his troublesome flame alchemy) was on the menu. If Father needs a problematic person gone, why not send them to Gluttony's stomach? If another homunculus can keep the target busy, Gluttony will get his chance to strike. He can do this by opening his chest to reveal his true mouth, and it can instantly "eat" anything in front of him, from houses and cars to trains and the ruins of Xerxes. Gluttony is like a walking eraser.
Gluttony also served as a testbed for a false Portal of Truth that Father tried to create because there was no way Father would try that on his other homunculus minions -- let alone on himself. Gluttony was a fine test subject, though the experiment was largely a failure. So, that false portal was repurposed for erasing evidence of Father's schemes, as described above, and it very nearly disposed of Ed and Ling for good. Only by using Xerxian runes, Envy's Philosopher's Stone and human transmutation could Ed open the portal again and escape Gluttony's stomach realm.
In short, Gluttony is a weak fighter because combat isn't what he's meant for; rather, that's what Lust and Wrath were assigned to do. Gluttony does what the other homunculi can't, and he does it best when he has backup.
