A few years back I gifted a sporterized Mauser 98 to a good friend who lives in White Plains, NY

Are there any special rules when gifting a weapon for either me in NJ or him in NY?

Is there any kind of trouble I can run into?

Excluding immediate family, a transfer is a transfer.  There are no special rules, even if you and your friend are joined at the hip.  You will have to do the transfer through and FFL.

Any person to person transfer where both parties live in different states, even an immediate family member, must go through a FFL.  The only exception is inheritance under federal and NJ law.  Don't know about other states.

So I guess I have to pray that no major crime is done with this WW1 M98. This will be a joke.