  In our previous work (E. Kogan, Phys. Rev. B {\bf 84}, 115119 (2011)) we
calculated RKKY interaction between two magnetic impurities in pristine
graphene using the Green's functions (GF) in the coordinate -- imaginary time
representation. Now we show that the calculations of the GF in this
representation can be simplified by using the Feynman's trick, which allows to
easily calculate RKKY interaction in gapped graphene. We also present
calculations of the RKKY interaction in gapped or doped graphene using the
coordinate -- imaginary frequency representation. Both representations,
corresponding to calculation of the bubble diagram in Euclidean space, have an
important advantage over those corresponding to calculation in Minkowskii
space, which are very briefly reviewed in the Appendix to the present work. The
former, in distinction to the latter, operate only with the convergent
integrals from the start to the end of the calculation.
