Does the Quran teach Unitarian Monotheism or Tawhid?
The Spirit is from God and part of Him, not part of creation. It is not a force. It can appear as a man and can be spoken to. He can also give life, a divine attribute.
Not only does the Spirit have divine attributes, he is also referred to using divine names. Therefore, the Spirit cannot be referring to Angel Gabriel.
In addition, Jesus was also described in the Quran as a Spirit from Allah and as the Word of Allah. The Word of Allah is distinct from Him and uncreated.