Veer Balbir Singh ji,
There are many references in Gurbani that promote belief in one Almighty God, and nothing else. While I am certainly not an expert in Gurbani, the first example that comes to my mind is Mool Mantra:
Ik Onkar, Sat Naam There is but One Almighty God who is the Supreme Truth.
To reinforce this point, our Gurus spoke many times about how a belief in God must be a belief ONLY in God. To "supplement" a belief in something other than the One Almighty was something that Nanak referred to as "duality".
As written in Sri Guru Granth Sahib ji, page 160:
Those who are in love with duality forget You, O God
If one goes to an astrologer, does that person do so because they want to make their life more difficult? Or is it because they are seeking some kind of guidance or assistance? If one consults a horoscope, does one do so to to be lied to? Or to be decieved? Or, does one go to seek some kind of truth?
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-sikhi-sikhism/3219-horoscopes-janam-kundli.html
What is truth? Nanak helps define it by first saying what is False, in Asa Di Var:
False is the king, false are the subjects; false is the whole world. False is the mansion, false are the skyscrapers; false are those who live in them. Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=3219 False is gold, and false is silver; false are those who wear them. False is the body, false are the clothes; false is incomparable beauty. False is the husband, false is the wife; they mourn and waste away. The false ones love falsehood, and forget their Creator. With whom should I become friends, if all the world shall pass away? False is sweetness, false is honey; through falsehood, boat-loads of men have drowned. Nanak speaks this prayer: without You, Lord, everything is totally false.
After that, Nanak continues in Asa Di Var to say what is true, or truth.
One knows the Truth only when the Truth is in his heart. The filth of falsehood departs, and the body is washed clean. One knows the Truth only when he bears love to the True Lord. Hearing the Name, the mind is enraptured; then, he attains the gate of salvation. One knows the Truth only when he knows the true way of life. Preparing the field of the body, he plants the Seed of the Creator. One knows the Truth only when he receives true instruction. Showing mercy to other beings, he makes donations to charities. One knows the Truth only when he dwells in the sacred shrine of pilgrimage of his own soul. He sits and receives instruction from the True Guru, and lives in accordance with His Will. Truth is the medicine for all; it removes and washes away our sins. Nanak speaks this prayer to those who have Truth in their laps.
And yes, Guru Nanak spoke strongly against astrologers/astrological teachings/astrology, because by their nature they cause a person to look to a source other than the One Almighty God for asnwers and inspiration. Guru Nanak warned about this in Sri Guru Granth Sahib ji, Page 904:
You calculate the auspicious days, but you do not understand that the One Creator Lord is above these auspicious days. He alone knows the way, who meets the Guru. When one follows the Guru's Teachings, then he realizes the Hukam of God's Command. Do not tell lies, O Pandit; speak the Truth. When egoism is eradicated through the Shabad, then one finds His Home, Reality. Calculating and counting, the astrologer draws the horoscope. He studies it and announces it, but he does not understand Reality. Understand, that the Gur-Shabad is above all. Do not speak of anything else; it is all just ashes.
I have to admit, the emphasis in the above stanza is mine. Please forgive my indulgence, but I believe that conclusion is absolutely timeless. Guru's Shabad is above everything. There is none higher.
The Rehat Maryada also spoke specifically against astrology...
Sikh Rehat Maryada
Article XVI - Living in Consonance with Guru's Tenets (Gurmat Rehni)
A Sikh's living, earning livelihood, thinking and conduct should accord with the Guru's tenets. The Guru's tenets are:
a) Worship should be rendered only to the One Timeless Being and to no god or goddess.
b) Regarding the ten Gurus, the Guru Granth Sahib and the ten Gurus' word alone as saviours and holy objects of veneration.
c) Regarding ten Gurus as the effulgence of one light and one single entity.
d) Not believing in caste or descent untouchabililty, Magic spells, incantation, omens, auspicious times, days and occasions, influence of stars, horoscopic dispositions...
The rehat goes on to speak of other rituals that are forbidden.
I will confess that I am not an expert neither in Gurbani, nor it's Vichaar. If I have made any mistakes, I would be most grateful if they are corrected.
Veer ji - have I been of any assistance to you?
Guru Fateh,
Amerikaur