Zinc oxide (ZnO) (K1) doped with rare earth metals (REM) such as lanthanum doped ZnO (K2), cerium doped ZnO (K3) and neodymium doped ZnO (K4) nanoparticles (NPs) were synthesized by green method using Gymnema sylvestre (G. sylvestre) leaves extract as reducing as well as capping agent and this method was also one of the alternatives to conventional physical and chemical methods. The synthesized K1, K2, K3 and K4 samples were characterized by X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDAX), Fourier transform infrared spectroscopy (FTIR), UV–visible spectroscopy, photoluminescence (PL) techniques and electron para magnetic resonance (EPR) spectroscopy. The K1, K2, K3 and K4 samples were tested against clinical pathogens such as gram positive G+ (Staphylococcus aureus and Streptococcus pneumoniae) and gram negative G− (Klebsiella pneumoniae, Shigella dysenteriae, Escherichia coli, Pseudomonas aeruginosa and Proteus vulgaris) bacterial strains using well diffusion method. The K2 sample shows higher antibacterial effect when compared to K1, K3 and K4 samples. In vitro cytotoxicity effect was analysed for A498 (human kidney carcinoma) cell line and normal vero (African monkey kidney) cell lines.