Are there any definitive references or clear evidence in Sri Guru Granth Sahib that explicitly state or suggest that the Sikh Gurus are considered God themselves?

Respected Brother/Sister,
Waheguru ji ka Khalsa, Waheguru ji ki Fateh!

In reply to your question, the undersigned wants to say that the First Guru Person, Guru Nanak Sahib had made it very clear, in his discussion with Yogis, that his Guru was Shabad when he said Shabad Guru Surat Dhun Chela (ਸਬਦ ਗੁਰੂ ਸੁਰਤਿ ਧੁਨਿ ਚੇਲਾ).

The Guru Granth Sahib starts with Jap Baani of First Guru Person, Guru Nanak Sahib, and the Baani starts with, what is called Mool Mantar, which defines the attributes of the One God (Akalpurakh), with whom a person can become one, with the help of Shabad Guru:-

ੴ (Ik Onkar) (One Universal Creator God);
ਸਤਿ ਨਾਮੁ (Sat Naam) (The Name Is Truth);
ਕਰਤਾ ਪੁਰਖੁ (Karta Purakh) (Creative Being Personified);
ਨਿਰਭਉ (Nirbhau) (No Fear);
ਨਿਰਵੈਰੁ (Nirvair) (No Enmity or Hatred),
ਅਕਾਲ ਮੂਰਤਿ (Akal Moorat) (Image of the undying);
ਅਜੂਨੀ (Ajuni) (Beyond Birth);
ਸੈਭੰ (Saibhang) (Self-existstant);
ਗੁਰ ਪ੍ਰਸਾਦਿ (Gur Parsad) (By Guru’s Grace).

Therefore, the First Guru Person, Guru Nanak Sahib, himself had made it very clear, in the very beginning of the Guru Granth Sahib, that there is one creator God and He is not in human form but resides everywhere and in every living being.

Therefore, Shabad was the Guru of Sikhs from the very beginning. Each of the Guru Persons, who were spiritually one with Akalpurakh, came with a mission from Him, and after fulfillment of the same, were replaced with the next Guru Person. Therefore, the bodies of Guru Persons are not considered as God in Sikhism.

Hope it helps. If you have any further questions, please do ask. If you find any deficiencies, please point out the same, for improvement in future.

Regards,
Your Brother