Context : Fossil galaxy groups are speculated to be old and highly evolved systems of galaxies that formed early in the universe and had enough time to deplete their L ^ { * } galaxies through successive mergers of member galaxies , building up one massive central elliptical , but retaining the group X-ray halo . Aims : Considering that fossils are the remnants of mergers in ordinary groups , the merger history of the progenitor group is expected to be imprinted in the fossil central galaxy ( FCG ) . We present for the first time radial gradients of single-stellar population ( SSP ) ages and metallicites in a sample of FCGs to constrain their formation scenario . We also measure line-strength gradients for the strongest absorption features in these galaxies . Methods : We took deep spectra with the long-slit spectrograph ISIS at the William Herschel Telescope ( WHT ) for six FCGs . The obtained spectra are fit with Pegase HR SSP models within the full-spectrum fitting package ULySS yielding SSP ages and metallicities of the stellar populations . We measure radial gradients of SSP ages and metallicities along the major axes . Lick indices are measured for the strongest absorption features to determine line-strength gradients and compare with the full-spectrum fitting results . Results : Our sample comprises some of the most massive galaxies in the universe exhibiting an average central velocity dispersion of \sigma _ { 0 } = 271 \pm 28 km s ^ { -1 } . Metallicity gradients are throughout negative with comparatively flat slopes of \nabla _ { [ Fe / H ] } = -0.19 \pm 0.08 while age gradients are found to be insignificant ( \nabla _ { age } = 0.00 \pm 0.05 ) . All FCGs lie on the fundamental plane , suggesting that they are virialised systems . We find that gradient strengths and central metallicities are similar to those found in cluster ellipticals of similar mass . Conclusions : The comparatively flat metallicity gradients with respect to those predicted by monolithic collapse ( \nabla _ { Z } = -0.5 ) suggest that fossils are indeed the result of multiple major mergers . Hence we conclude that fossils are not ’ failed groups ’ that formed with a top heavy luminosity function . The low scatter of gradient slopes suggests a similar merging history for all galaxies in our sample .