At the very beigining i would say the question is wrong:

Let us first make it very clear for once and for all that Guru nanak said
There is only ONE GOD for all
"Sabna Jiyan Ka Ik Datta So Main Visar Na Jaeen"
So the question " who is God in Sikhism?" arise,if there are more than one God, therefore, it is stupid to say who is your God or who is Mine God. There is no diepute nor it is a matter of discussion or argument or proving to any one. However, those who wish to tease others they raise such questions.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-sikhi-sikhism/8536-who-is-god-in-sikhism.html

In the world there are only two type of people for whom GOD matters
One who believes and the other who do not believe
The third Category is of DOUBTFUL Characters they are not stable hence they are neither believers nor non believers hence very dangerous and threat to the whole mankind. Because these are the people who are Hypocrates These are the people who are ungreatful.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=8536

Because those who are in doubt they receive neither Ram nor Maya. "Dubidha mein dono Gayo Maya Milli Na Raam"

I love non believers because they atleast believe in themselves but not anything else and they have great faith in self, if not GOd. But the Doubtful has "NO FATH"
neither in self nor in God.

People of faith in self or God are most relevant at least his faith is without doubt that here is no GOd. Still he lives with some firm faith.

Doubtful is dangerous and your Question seems to me coming from a Doubtful Character.
God is with in you and "ander Vaseh Bahar Bhi Ohi"
and he is "rav reha sarbatra Mein" "Har Ghat Ram Bole, Ram bina Kuch Bole Na" "Deh Matti Bole Pawan Bujh re giani Muva Kaun"

If your Question is to know by what Name do Sikhs Know God?

For answer Read GUru Nanak.
He has millions of names but he is one "Anek hai Phir Ek Hai"

Dear Friends Sikh is a seeker and to seek the first requirement is :
Unwavering Faith
Submit to Thy Will
By Simran Connect to Him
Like "Babe Chakkiyan Challaiyan" Like Baba Nanak by Simran let the Grinders revolve and grind flour in the Babur's Jail. Your all work will be done by Simran - Read Sukhmani Sahib.

I have full faith.
This is the power of Simran.

Dhan Nirankar Dhan baba Nanak
HP Luthera