Gardnerella vaginalis 6.0 log cells/ml on swab 3 weeks after completing 5 day vaginal metronidazole gel treatment. now prescribed vaginal clindamycin cream 7 days. is this likely to work better? what would be a 3rd line treatment in case of failure?

Gardnerella itself is not abnormal in the vagina. It is useful for diagnosis because it grows to increased amounts in bacterial vaginosis, but so do many other vaginal bacteria; G. vaginalis is not itself the cause. The treatment of choice is metronidazole by mouth, not vaginally. If the clindamycin doesn't work, stop all treatment and see a gyn who specializes in infectious diseases. Good luck.