I don't think there is a direct reference to Christians or Jews in Gurbani but their holy books have been mentioned; but indirectly. The word "Kateb" includes not only Quran but also Bible and Torah.
Some people argue that Guru Sahib was not aware of these religions since they had not become popular in India at that time. This statement is not correct. Siri Guru Nanak Dev jee travelled extensively throughout the world and must have known about these religions but did not comment on them because Guru Sahib ( it seems like ) did not think these religions were even worthy of that. A proof that Guru Sahib knew about these religions is the following shabads of Bhai Gurdas jee, that clearly mentions both Christians and Jews:
ਬਹੁ ਸੁੰਨੀ ਸ਼ੀਅ ਰਾਫਜ਼ੀ ਮਜ਼ਹਬ ਮਨਿ ਭਾਣੇ॥bahu su(n)nee sheea raafazee mazehab man bhaanae||
Many people are there of attractive sects of Sunnis, Sias and Rafazis.
ਮੁਲਹਿਦ ਹੋਇ ਮੁਨਾਫ਼ਕਾ ਸਭ ਭਰਮਿ ਭੁਲਾਣੇ॥mulehidh hoe munaakaa sabh bharam bhulaanae||
Many hypocrites becoming atheists and deluded by illusions go on wandering here and there.
ਈਸਾਈ ਮੂਸਾਈਆਂ ਹਉਮੈ ਹੈਰਾਣੇ॥eesaaee moosaaeeaaa(n) houmai hairaanae||
Followers of Jesus and Moses are also many who are confounded in their own pride.
ਹੋਇ ਫਿਰੰਗੀ ਅਰਮਨੀ ਰੂਮੀ ਗਰਬਾਣੇ॥hoe fira(n)gee aramanee roomee garabaanae||
Inhabitants of the European countries are also trapped in pride.
ਕਾਲੀ ਪੋਸ ਕਲੰਦਰਾਂ ਦਰਵੇਸ ਦੁਗਾਣੇ॥kaalee pos kala(n)dharaa(n) dharavaes dhugaanae||
Some are black clad recluses and dervishes wearing bunches of cowaries around their arms who move about here and there.
ਗੁਰੁ ਸਿਖ ਰੋਮ ਨ ਪੁਜਨੀ ਗੁਰ ਹਟਿ ਵਿਕਾਣੇ ॥11॥gur sikh rom n pujanee gur hatt vikaanae ||11||
They all are not equal to even a hair of those Gurskihs who have sold themselves at the hands of the Guru.(11)
Another Vaar of Bhai Gurdaas jee goes as follows:
ਕਿਤੜੇ ਸੁੰਨੀ ਆਖੀਅਨਿ ਕਿਤੜੇ ਈਸਾਈ ਮੂਸਾਈ॥kitharrae su(n)nee aakheean kitharrae eesaaee moosaaee||
Many are Christians, Sunnis and followers of Moses.
ਕੇਤੜਿਆ ਹੀ ਰਾਫਜੀ ਕਿਤੜੇ ਮੁਲਹਿਦ ਗਣਤ ਨ ਆਈ॥kaetharriaa hee raafajee kitharrae mulehidh ganath n aaee||
Many are Rafizis and Mulahids (those who do not believe in the day of judgement).
ਲਖ ਫਿਰੰਗੀ ਇਰਮਨੀ ਰੂਮੀ ਜੰਗੀ ਦੁਸ਼ਮਨ ਦਾਈ॥lakh fira(n)gee eiramanee roomee ja(n)gee dhushaman dhaaee||
Millions are firangis (Europeans), Arminis, Rumis and other warriors fighting the enemy.
ਕਿਤੜੇ ਸਈਯਦ ਆਖੀਅਨਿ ਕਿਤੜੇ ਤੁਰਕਮਾਨ ਦੁਨਿਆਈ॥kitharrae seeyadh aakheean kitharrae thurakamaan dhuniaaee||
In the world many are known by the names of Sayyads and Turks.
ਕਿਤੜੇ ਮੁਗਲ ਪਠਾਣ ਹਨਿ ਹਬਸੀ ਤੈ ਕਿਲਮਾਕ ਅਵਾਈ॥kitharrae mugal pat(h)aan han habasee thai kilamaak avaaee||
Many are Mughals, Pathans, Negroes and Kilmaks (followers of Solomon).
ਕੇਤੜਿਆਂ ਈਮਾਨ ਵਿਚ ਕਿਤੜੇ ਬੇਈਮਾਨ ਬਲਾਈ॥kaetharriaaa(n) eemaan vich kitharrae baeeemaan balaaee||
Many are spending honest life and many live by dishonesty.
ਨੇਕੀ ਬਦੀ ਨ ਲੁਕੈ ਲੁਕਾਈ ॥16॥naekee badhee n lukai lukaaee ||16||
Even then, the virtue and evil cannot remain hidden.(16)
Gurbani mentions Rama, Krishna, Shiva, Brahma, Buddha but does not mention Mahavira (leader of Jain faith), Muhammad, Moosa (Moses), and Isa Masih (Jesus Christ). To me it tells how unimportant they were in the eyes of Guru Sahib.
On one hand Guru Sahib mentions so many old Bhagats but totally ignores Ashtavakra (one of the greatest leaders of Advaita Vedanta). This tells me that Guru Sahib did not see Advaita as even worthy of mention. Ashtavakra at one time was the Guru of Raja Janak who is mentioned because he must have changed his Bhagti to that of Prema Bhagti (devotional Bhagti) as opposed to the dry Advaita Vedanta philosophy that he subscribed to when he was the disciple of Ashtavakra.
Kulbir Singh
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥