#Yes!
Yes!
Well, sort of. While the other answers are correct in saying these ultra-light gasses, or even gaseous vacuum would have a minimal effect, I felt there was something overlooked.
Atmsophere
#Atmsophere TheThe pockets of gas could certainly add a slight lifting element, reducing weight marginally, barely noticeably infact. But what about the atmosphere itself? The buoyancy of the gas (and armor) would be significantly increased relative to the heaviness of the surrounding atmosphere. Take Handwavium-X, its a super dense gas; about 1/4th the density of water! Your planet's atmosphere is made of a great deal of Handwavium-X, your armor-wearing beings could even process it in a similar way to how we breath oxygen, or at the very least have it be non-consequential similar to the nitrogen and various other gasses in our system.
Combine this with Handwavium-Y (or Vacuum Gas) filling the gaps in your armor, and your armor becomes relatively light. Of course the other given concerns of extra bulk, armor weakpoints, compromised integrity, reliance on it being sealed, etc. might cause issues. Those aside, it is theoretically doable given the constraints (or lack-thereof) you provided.