  We present linear polarimetry for seven hydrogen-poor superluminous
supernovae (SLSNe-I). For SN 2017gci, for which we present two epochs of
spectropolarimetry at +3 d and +29 d post-peak in rest frame, accompanied by
four epochs of imaging polarimetry up to +108 d. The spectropolarimetry at +3 d
shows increasing polarisation degree P towards the redder wavelengths and
exhibits signs of axial symmetry, but at +29 d P=0 throughout the spectrum
implying that the photosphere of SN 2017gci evolved from a slightly aspherical
configuration to a more spherical one in the first month post-peak. However, an
increase of P to 0.5% at +55 d accompanied by a different orientation of the
axial symmetry compared to +3 d implies the presence of additional sources of
polarisation at this phase. The increase in polarisation is possibly caused by
interaction with circumstellar matter as already suggested by a knee in the
light curve and a possible detection of broad Ha emission. We also analysed the
sample of all 16 SLSNe-I with polarimetry to date. The data taken during the
early spectroscopic phase show consistently low P indicating spherical
photospheres. No clear relation between the polarimetry and spectral phase was
seen when the spectra resemble Type Ic SNe during the photospheric and nebular
phases. The light curve decline rate also shows no clear relation with the
polarisation properties. While only slow-evolving SLSNe-I have shown non-zero
P, the fast-evolving ones have not been observed at sufficiently late times to
conclude that none of them exhibit changing P. However, the four SLSNe-I with
increasing polarisation degree also have irregular light curve declines. For up
to half of them, the photometric, spectroscopic and polarimetric properties are
affected by CSM interaction. As such CSM interaction clearly plays an important
role in understanding the polarimetric evolution of SLSNe-I.
