Is a neutral person in a religious war equal to enemy, according to GITA?
Gita is quite clear that one must oppose the wicked and fight on the side of dharma. There is no room for neutrality in a war against adharma. The concrete example is that of Arjuna. He refused to fight against adharma on personal grounds. Sri Krishna chastised him for such unmanliness.
O Arjuna! Whence has this loathsome stupidity come upon you in this crisis? It (this attitude) is unworthy of a noble personage.** It is a bar to heaven and a cause of much disrepute.**
O Partha! Yield not to unmanliness! It befits thee not. Abandoning this base faint-heartedness, rise up, O dreaded hero!
Hindu dharma usually recommends nonviolence. However, there may be occasions when violence is necessary for defence of dharma.
Tuladhara said, ‘O Jajali, I know morality, which is eternal, with all its mysteries. It is nothing else than that ancient morality which is known to all, and which consists of universal friendliness, and is fraught with beneficence to all creatures. That mode of living which is founded upon a total harmlessness towards all creatures or (in case of actual necessity) upon a minimum of such harm, is the highest morality.’
(Mahabharata Santi Parva Section CCLXII)
A neutral person in a war against adharma is not an enemy but he is definitely misguided.
Gita directly doesn't discuss about the particular term "neutrality" during the war. However, it does [repeatedly] discuss that for whatever reasons abandoning the righteous war (Dharma Yudhha) results in sin.
BG 2.33 - अथ चैत्त्वमिमं धर्म्यं संग्रामं न करिष्यसि। ततः स्वधर्मं कीर्तिं च हित्वा पापमवाप्स्यसि।
On the other hand, if you will not fight this righteous battle, then forsaking your own duty and fame, you will incur sin.
BG 2.34 - People also will speak of your unending infamy. And to an honoured person infamy is worse than death.
Famous example of being neutral in the religious war was Krishna's own half brother BalarAma, who didn't participate in any way during the war.
In famous BRC Mahabharata, in the episode of Duryodhana's defeat, it's illustrated nicely in this small scene between Krishna & BalarAma. Though it may not be exactly according to the scripture of Mahabharata, but it conveys the sentiment very well.
Is a neutral person in a religious war equals to enemy, according to GITA?
No according to Shrimad Bhagavad Gita a neutral person is actually best among yogi's This is mentioned by Shree Krishna in Chapter 6 - Verse 9. Below is the English Translation Of Sri Shankaracharya's Sanskrit Commentary By Swami Gambhirananda.
साधुष्वपि च पापेषु समबुद्धिर्विशिष्यते।।6.9।।
sādhuṣv api ca pāpeṣu sama-buddhir viśiṣyate
6.9 The first line of the verse beginning with 'benefactor,' etc. is a single compound word. Visisyate, he excels, i.e. he is the best among all those who are established in Yoga-(a different reading is vimucyate, he becomes free); sama-buddhih, who has sameness of view, i.e. whose mind is not engaged with the estion of who one is and what he does; with regard to a suhrd, benefactor-one who does some good without consideration of return; mitram, a friend, one who is affectionate; arih, a foe; udasinah, a neutral, who sides with nobody; madhyasthah, an arbiter, who is a well-wisher of two conflicting parties; dvesyah, the hateful, who is repulsive to oneself; bandhuh, a relative;- to all these as also sadhusu, with regard to good people, who follow the scriptures; api ca, and even; papesu, sinners, who perform prohibited actions-with regard to all of them. Therefore, to acire this excellent result-
A person is considered still further advanced when he regards honest well-wishers, affectionate benefactors, theneutral , mediators, the envious, friends and enemies, the pious and the sinners all with an equal mind.
Here as desired by you the two or more conflicting parties can be regarded as those whom are fighting religious war. So no the neutral person who sees no difference between differentationing parties (even religious), and is benefactor to all is considered as advance yogi.