Probably involved in guanidinium transport (PubMed:27989440). In vitro, confers resistance to a broad range of toxic compounds such as cationic dyes, neutral and anionic antimicrobials (PubMed:10735877). The efflux pump is composed of GdnC and GdnD. Transcriptionally regulated by guanidine, via a guanidine-sensing riboswitch. Deletion of both gdnC and gdnD leads to a decrease in the concentration of guanidine required to inhibit bacterial growth. Belongs to the drug/metabolite transporter (DMT) superfamily. Small multidrug resistance (SMR) (TC 2.A.7.1) family. YkkC/YkkD subfamily.