[mothered]

does gross weight really matter when shipping the return back to amazon?

Obviously the gross weight will be recorded during transit, hence there must not be a significant variance with the original shipment.

should it be the same weight as it came in?

Yes.

Given you're using the same packaging to send the return, you'll be focusing on matching the Item weight.

do I have to seal the amazon box it came with also? or is it ok that just the laptop box is sealed to perfection?

The laptop box will suffice but to play It safe, where possible, reseal both.

was planning on acquiring a postal scale to make sure weight is correct, is this necessary?

Every SE'er should have a postal/kitchen scale on hand.

Be sure It's a good quality scale. As such, you'll have peace of mind that It will display an accurate reading.