  We present and discuss the optical spectro-photometric observations of the
nearby (z=0.087) Type I superluminous supernova (SLSN I) SN 2017gci, whose peak
K-corrected absolute magnitude reaches Mg=-21.5 mag. Its photometric and
spectroscopic evolution includes features of both slow and of fast evolving
SLSN I, thus favoring a continuum distribution between the two SLSN-I
subclasses. In particular, similarly to other SLSNe I, the multi-band light
curves of SN 2017gci show two rebrightenings at about 103 and 142 days after
the maximum light. Interestingly, this broadly agrees with a broad emission
feature emerging around 6520 A after 51 days from the maximum light, which is
followed by a sharp knee in the light curve. If we interpret this feature as
Halpha, this could support the fact that the bumps are the signature of late
interactions of the ejecta with a (hydrogen rich) circumstellar material. Then
we fitted magnetar and CSM-interaction powered synthetic light curves onto the
bolometric one of SN 2017gci. In the magnetar case, the fit suggests a polar
magnetic field Bp = 6 x 1e14 G, an initial period of the magnetar Pinitial=2.8
ms, an ejecta mass Mejecta=9 Msun and an ejecta opacity k = 0.08 cm g^{-1} . A
CSM interaction scenario would imply a CSM mass of 5 Msun and an ejecta mass of
12 Msun. Finally, the nebular spectrum of phase 187 days was modeled, deriving
a mass of 10 Msun for the ejecta. Our models suggest that either a magnetar or
CSM interaction might be the power sources for SN 2017gci and that its
progenitor was a massive (40 Msun) star.
