Endurance provides the user the ability to convert their morale into physical energy and stamina. It is demonstrated by Gigantomachia that the more excited the user is, or the more driven they are to accomplishing a task, the stronger and more invigorated they become.
This natural sense of energy and stamina appears to also allow the user to very easily be able to hold and utilize multiple Quirks at once, without being affected by the strain on the body or mind, or having to be surgically remodeled like the Nomu.
This Quirk acts as the crux to Gigantomachia's proposed usefulness as a bodyguard, as his unending loyalty to his master (initially All For One and later Tomura Shigaraki once he proved himself) gives him the motivation to please them at any cost, thus increasing his strength and resilience.
The apparent downside to this Quirk is that, eventually, the user's energy can be worn out through a repeated onslaught of attacks without reinforcement of their morale. Additionally, due to the need to be highly moralized regarding the object of loyalty, plus Gigantomachia's apparent need to physically hear his master's issuing a command, when the user is on standby waiting for a command, they become inactive and unable to properly assist, like when the Gunga Mountain Villa was initially attacked where Gigantomachia did not immediately act defensively. The user also becomes significantly weaker until ordered, as was the case where Gigantomachia was more susceptible to attacks like Best Jeanist's binds until commanded by his master to act (though the addition of a sedative's effects taking hold in that moment might have added to this).
- My Hero Academia Manga: Vol. 29, Omake