What do we say about Guru Gobind Singh Ji saying that whoever calls him God will go to Hell?

There is no doubt that Guru and Waheguru are one and the same. Guru is a form of Waheguru; formed to play the crucial role of freeing the Jeevs from Maya.

How do you explain the lines like:
ਜੇ ਹਮ ਕੋ ਪਰਮੇਸਰ ਉਚਰਿ ਹੈ ॥ ਤੇ ਸਭ ਨਰਕ ਕੁੰਡ ਮਹਿ ਪਰਿ ਹੈ ॥
ਮੋ ਕੌ ਦਾਸੁ ਤਵਨ ਕਾ ਜਾਨੋ ॥ ਯਾ ਮੈ ਭੇਦ ਨ ਰੰਚ ਪਛਾਨੋ ॥ ੩੨ ॥
jae ham ko paramaesar ouchar hai | thae sabh narak kundd mehi par hai |
mo ka dhaas thavan kaa jaano | yaa mai bhaedh n ranch pashaano | 32 |

Translation:- “Those who call me God, will fall into the pit of hell.
Regard me as a humble servant of the Lord and have no doubt about it.”

The lines really are written by Guru Gobind Singh Ji to avoid Sikhs to indulge in the pooja of “Guru Gobind Singh Ji” instead of worshipping the Guru-jyot (Guru Light) in Guru Gobind Singh Ji.
The previous avatars like Krishna and Rama did good works but ended up becoming gods themselves and having people worship them. This has been highlighted in Bachitter Natak by Siri Dasmesh jee. In other religions they say “Jai Raam jee kee” or “Jai Sita Raam” or “Har Har Mahadev” (referring to Shiv jee). But in Sikh faith, Guru Sahib never asked us to shout “Guru Gobind Singh Zindabad” or “Guru Gobind Singh kee jai”. Instead, we are trained to say “Sat Sri Akaal” and thus paying homage to Waheguru alone. We don’t japp the Naam of any Guru and nor it is instructed in Gurbani to do so. We are instructed to japp Naam of Waheguru. This is the greatness of the Ten Guru Sahibaan who were themselves Guru-form of Waheguru but still instructed Sikhs to japp Naam of Waheguru only.

These lines of Guru Gobind Singh Ji, quoted in the beginning, are true in the sense that whole of Waheguru, who pervades the whole universe was not restricted in the human body of Guru Gobind Singh Ji as Hindus think he was in the body of Siri Krishna. Guru Arjun Dev Ji has said it very clearly that let that mouth burn that says Thakur comes in joni (takes birth).

ਸੋ ਮੁਖੁ ਜਲਉ ਜਿਤੁ ਕਹਹਿ ਠਾਕੁਰੁ ਜੋਨੀ ॥੩॥
So mukẖ jala­o jiṯ kaheh ṯẖākur jonī. ||3||
Translation:- “Let that mouth be burnt, which says that our Lord and Master is subject to birth. ||3||
(Ang 1136)

Waheguru (as a whole) is Ajooni i.e. never takes birth. Our Guru Sahibaan were a form of Waheguru i.e. the Guru Form of Waheguru, created to bring the lost jeev-aatmas on right path. The whole of universe has been given to the Guru-form of Waheguru as per the following line:
ਸ੍ਰੀ ਗੁਰੂ ਰਾਜੁ ਅਬਿਚਲੁ ਅਟਲੁ ਆਦਿ ਪੁਰਖਿ ਫੁਰਮਾਇਓ ॥੭॥
Sarī gurū rāj abicẖal atal āḏ purakẖ furmā­i­o. ||7||
Translation:- “The sovereign rule and kingdom of the Great Guru (Guru Nanak) is unchanging and permanent, so declared Aad Purakh, the Primal Lord God. ||7||”
(Ang 1390)