  We have performed magnetic torque and resistivity measurements on a
centrosymmetric skyrmion-host GdRu2Si2, in which the dominant magnetic
interaction leading to skyrmion formation is under debate. We observe both the
de Haas-van Alphen and Shubnikov-de Haas oscillations in the forced
ferromagnetic phase. The angular dependence of the quantum oscillation
frequencies can be reproduced by the ab-initio calculation. The de Haas-van
Alphen oscillation is also observed in the double-Q phase with a different
frequency to that in the forced ferromagnetic phase, indicating a Fermi surface
reconstruction due to the coupling between localized spins and conduction
electrons. Based on these experimental findings, the magnetic interactions in
this system are discussed.
