Also, there are new and old comments about bodies sinking and later surfacing due to gas build up. I expect that this scenario is limited to the common case of shallow water. Once something sinks in several thousand meters of water, it would take a LOT of gas to make it buoyant again,, due to the high ambient pressure.
Almost certainly yes. As the body sinks, the gas filled parts (ie lungs and gastrointestinal tract) will be compressed. The gas may or may not be expelled - doesn't really matter. But it must be compressed and the body will become more dense due to decreased volume (or if you prefer, less buoyant as it will displace less water).
Once this has happened (and there is no magic depth here - it is a continuum), then it is highly unlikely that subsequent gas build up will be sufficient to increase the volume of the body to that required to overall become less dense than water. There may well be gas production, but it will be exposed to the same ambient pressure and hence will not occupy much volume (or be able to displace much water).