So
‘He or she’ versus ‘they’ | Oxford Dictionaries technically for those that don't know God's gender they should say he/she, or, he or she, etc.
But for those that believe in Jesus. The answer is quite simple. Jesus referred to God as his Father.
John 14 8-21
8 Philip saith unto him, Lord, show us the Father, and it sufficeth us.
9 Jesus saith unto him, Have I been so long time with you, and yet hast thou not known me, Philip? he that hath seen me hath seen the Father; and how sayest thou then, Show us the Father?
10 Believest thou not that I am in the Father, and the Father in me? the words that I speak unto you I speak not of myself: but the Father that dwelleth in me, he doeth the works.
11 Believe me that I am in the Father, and the Father in me: or else believe me for the very works' sake.
12 Verily, verily, I say unto you, He that believeth on me, the works that I do shall he do also; and greater works than these shall he do; because I go unto my Father.
13 And whatsoever ye shall ask in my name, that will I do, that the Father may be glorified in the Son.
14 If ye shall ask any thing in my name, I will do it.
15 If ye love me, keep my commandments.
16 And I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever;
17 Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you.
18 I will not leave you comfortless: I will come to you.
19 Yet a little while, and the world seeth me no more; but ye see me: because I live, ye shall live also.
20 At that day ye shall know that I am in my Father, and ye in me, and I in you.
21 He that hath my commandments, and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him, and will manifest myself to him.
So going off that I'd say God is masculine so "he" is just fine. Gender identity is not determined by primary or secondary sex characteristics as any trans person will tell you. God has asked us to refer to Him as a he, as told to us by Jesus. So that means He identifies as male. If you truly respect a person's right to be called by their preferred pronoun then "he/him/his" is the only grammatically and politically correct way to refer to God.