I'm not Indian, but I've got a few Indian friends and have attended a few Indian weddings as well. From what I've seen, there's no "traditional" gift - most of the gifts I've seen are something which would be useful for the couple or their house, eg a dinner set, clothes, jewelery, maybe even a gadget or appliance. But this really depends on the couple - younger couples these days may explicitly say no to any physical gifts (because they may just end up becoming clutter) and would prefer accepting cash or gift cards instead. So if you're close to your friend, I'd say ask them upfront what sort of gift they would prefer or if there's any type of gift they wouldn't want - and that should help you narrow it down. Beware that they may say not to worry about bringing a gift and just your presence alone would be a gift (I've heard this dialog a few times...), but regardless, you shouldn't back down, and insist that you will gift them something irrespective of what they say, so this should make them back down if they're being stubborn. :)
As someone else said, a Polish gift might be a good idea - doesn't have to be a "useful" thing, but something symbolic to remember you would be nice.
My interaction with my neighbors is just to the extent of saying "hi" when we bump into each other and that's it. They have a friendly dog and I love dogs, so I did offer them that I could dog sit for them if they ever needed it, but they never took up the offer. I've spoken way more to their dog than I ever did to them.
Oh well. ¯\_(ツ)_/¯
I believe the era of "love thy neighbour as thyself" is long gone. Back in the day there were many reasons to interact with your neighbors, such as for keeping up with the local news and gossip, borrowing rations when you've run out of stuff, or keeping an eye out on your place when you're away etc. But these days, in the era of the Internet, home deliveries and security cameras, there's very little need to interact with your neighbors. Which is a bit sad, but I guess that's the way the world works now.