Obtaining list

Via name:

Unity::il2cppArray<Unity::CGameObject*>* m_pObjects = Unity::Object::FindObjectsOfType<Unity::CGameObject*>("UnityEngine.GameObject");

Via pre-cached SystemType:

Unity:il2cppObject* m_pSystemType = nullptr;

void InitSystemTypes()
{
    Unity::il2cppClass* m_pSystemTypeClass = IL2CPP::Class::Find("UnityEngine.GameObject");
    m_pSystemType = IL2CPP::Class::GetSystemType(m_pSystemTypeClass);
}

Unity::il2cppArray<Unity::CGameObject*>* m_pObjects = Unity::Object::FindObjectsOfType<Unity::CGameObject*>(m_pSystemType);

Looping through list:

for (uintptr_t u = 0U; m_pObjects->m_uMaxLength > u; ++u)
{
    Unity::CGameObject* m_pObject = m_pObjects->m_pArray[u];
}