I don't have much first-hand experience, since I don't speak Japanese. However, I have read some phonetic studies that have some relevant information.
As naruto's answer explains, the basic pronunciation of ッ is not a separate sound, but a lengthening of the following consonant sound. When it occurs before p, t, k, the pronunciation is generally transcribed phonetically as a long or "geminate" consonant, written in the IPA as [pp], [tt], [kk] or [pː], [tː], [kː]. (This is not the same thing as a glottal stop, although it may sound similar.)
So the pronunciation we would expect for ッ before b, d, g is simply [bb], [dd], [ɡɡ] or equivalently, [bː], [dː], [ɡː]: a long (or geminate) voiced plosive.
However, there is some linguistic literature that indicates that voiced geminate consonants in Japanese are sometimes "devoiced", making them sound more like [pː], [tː], [kː]. Here are some papers on this that I found:
I think I hear devoicing in the first pronunciation of "バッグ" on Forvo (by gordon3155), although I'm not certain.