Sorry, I don't know the answer to that. I don't have either a G12 or a filter tube.
answered on July 8, 2013
This case fits the G15 very snugly. I don't have a filter tube, but in looking at pictures, my guess would be no. Also this case is specifically for the G15. I don't have a G12, but I'd definitely does not fit my G9.
Hope this helps.
Sorry, I don't have a G12 so can't answer your question. Good luck finding an answer!