Not supposed to smell at all. Could have anaerobic pockets somewhere in the system. Do your GB's drain completely? Does the FT turnover within one hour? Are you missing any fish?
Sometimes outlet pipes are partially clogged with decaying debris/waste, sometimes pockets due to root balls and cause waste accumulation develop in GB's where there is poor water(oxygen) movement so i do hope you do regular maintenance on your system, with these conditions in mind.
Worms do much work in AP helping to release nutrient that's more easily absorbed by plants but, i find, they tend to avoid these anaerobic areas, and if conditions persist it may ultimately affect their propagation cycles, not to mention the system itself.