Common sense suggests that if the law says it's illegal to wear it as a civilian then "but they're available for sale on Amazon!" probably would not be an effective defence in court.
is it really going to get a civilian in trouble if he wears any kind of Army camouflage for a trip?
Hard to know for sure.
It is only the beret and badges that distinguish the real armed forces personnel from someone wearing a fatigue as a fashion-statement therefore making it very difficult to distinguish a real armyman from the fake.
Which is why the Indian Army issued guidelines on Friday requesting civilians not to wear the combat dress of the army and asked shopkeepers not to sell them too.
The guidelines are to be followed across the country. The Army has now labelled the selling of such clothing and equipment “illegal”.
The guidelines not to wear “combat-pattern” dresses extend to private security agencies, police and other central forces too.
The Army has said that wearing such dresses “leads to false alarms”.
Indian Army Has Made It Illegal For Civilians To Wear Army-Style Dress, Topyaps 9 Jan 2016
By "false alarms" I understand them to mean, "if you're in full camo then you might be mistaken for a terrorist and shot".
If it's just a rucksack and the rest of your clothing is very obviously non-military then it's hard to see you really being mistaken for some kind of paramilitary while wearing it, but it's not impossible that a punctilious official might have a problem with it.
The army seems to argue that it's your patriotic duty not to wear camo as a civilian:
The Army appealed and requested the public to adhere to the guidelines in national as well as their own interest.
Avoid wearing 'Army-pattern' dress, Army tells civilians, Times of India 8 Jan 2016
... so that could provide you with a decent reason to return the present without causing offence.
Or you could just keep it but not use it.
Update: commenter @RedBaron (thank you!) adds that this is covered by Section 140 of the Indian Penal Code:
- Wearing garb or carrying token used by soldier, sailor or airman.—Whoever, not being a soldier, sailor or airman, in the Military, Naval or Air service of the Government of India, wears any garb or carries any token resembling any garb or token used by such a soldier, sailor or airman with the intention that it may be believed that he is such a soldier, sailor or airman, shall be punished with imprisonment of either description for a term which may extend to three months, or with fine which may extend to five hundred rupees, or with both.