We present new spectroscopic observations obtained with GMOS @ Gemini-S of a sample of 25 H ii regions located in NGC 55 , a late-type galaxy in the nearby Sculptor group . We derive physical conditions and chemical composition through the T _ { e } -method for 18 H ii regions , and strong-line abundances for 22 H ii regions . We provide abundances of He , O , N , Ne , S , Ar , finding a substantially homogenous composition in the ionised gas of the disc of NGC 55 , with no trace of radial gradients . The oxygen abundances , both derived with T _ { e } - and strong-line methods , have similar mean values and similarly small dispersions : 12+ \log ( O/H ) =8.13 \pm 0.18 dex with the former and 12+ \log ( O/H ) =8.17 \pm 0.13 dex with the latter . The average metallicities and the flat gradients agree with previous studies of smaller samples of H ii regions and there is a qualitative agreement with the blue supergiant radial gradient as well . We investigate the origin of such flat gradients comparing NGC 55 with NGC 300 , its companion galaxy , which is also twin of NGC 55 in terms of mass and luminosity . We suggest that the differences in the metal distributions in the two galaxies might be related to the differences in their K-band surface density profile . The flatter profile of NGC 55 probably causes in this galaxy higher infall/outflow rates than in similar galaxies . This likely provokes a strong mixing of gas and a re-distribution of metals .