Missionaries, and few scholars question it's authenticity.
Though controversial Scripture among Sikhs, following section is from that portion of scripture which even missionaries believe was written by Gurmukh(May be Guru Gobind Singh). Harjinder Singh Dilgeer in a meeting with Prabhdeep Singh Tiger Jatha (Missionary) believes it to be written by Gurmukh Poet.
Now come to topic,
Guru Gobind Singh ji was fond of hunting as per Dasam Granth Sahib.
It is mentioned:
ਅਥ ਰਾਜ ਸਾਜ ਕਥਨੰ ॥
ਰਾਜ ਸਾਜ ਹਮ ਪਰ ਜਬ ਆਯੋ ॥ ਜਥਾ ਸਕਤਿ ਤਬ ਧਰਮ ਚਲਾਯੋ ॥
When I obtained the position of responsibility, I performed the religious acts to the best of my ability.
ਭਾਂਤਿ ਭਾਂਤਿ ਬਨਿ ਖੇਲ ਸਿਕਾਰਾ ॥ ਮਾਰੇ ਰੀਛ ਰੋਝ ਝੰਖਾਰਾ ॥੧॥
I went hunting various kinds of animals in the forest and killed bears, nilgais (blue bulls) and elks.1.
In Charitar 71, which is considered as autobiography of Guru Gobind Singh among Mainstream Sikhs, Author writes:
ਖਿਲਤ ਅਖੇਟਕ ਸੂਕਰ ਮਾਰੇ ॥ ਬਹੁਤੇ ਮ੍ਰਿਗ ਔਰੈ ਹਨਿ ਡਾਰੇ ॥
ਪੁਨਿ ਤਿਹ ਠਾ ਕੌ ਹਮ ਮਗੁ ਲੀਨੌ ॥ ਵਾ ਤੀਰਥ ਕੇ ਦਰਸਨ ਕੀਨੌ ॥੩॥
ਦੋਹਰਾ ॥ ਤਹਾ ਹਮਾਰੇ ਸਿਖ੍ਯ ਸਭ ਅਮਿਤ ਪਹੂੰਚੇ ਆਇ ॥ ਤਿਨੈ ਦੈਨ ਕੋ ਚਾਹਿਯੈ ਜੋਰਿ ਭਲੋ ਸਿਰਪਾਇ ॥੪॥
Guru Gobind Singh mentioned that he had killed:
5) Wild Boars
The animals mentioned above were much in number during Guru's time and Physical, agricultural losses are associated with them.
But it is Jeev Hatya. and Jeev Hatya is sinful act and Self Centered.