Can I use a 16 gauge nail gun for framing lumber?
A 16ga nailer is considered a finish nailer, and really isn't meant for joining 2x4s.  They are primarily used to attach 3/4" boards in the form of trim to studs or other materials.  The small diameter nail just doesn't have the holding strength to make framing  for a bench or anything else like that.

You would need a framing nailer for that job, but they are big, loud, and not unlike firing a gun into the boards.  They are heavy and have a lot of kick.

What I would suggest is an impact driver and 2 1/2" or 3" decking screws.  A normal drill needs a lot of force from the user to drive a long screw, but impact drivers make the job easier.  If it still takes more force than you can apply, you can pre-drill the holes before screwing.  For screwing at an angle (like toe nailing), pre-drilling makes it much easier.  In fact, if you're ok with pre-drilling every hole, a standard drill might not be too hard for you to use.



Impact Driver Example

There are many brands, and I chose the above at random.  You can pay more or less so shop around.  You don't need anything heavy duty.