We report on a study of the structural, magnetic, and superconducting properties of $\mathrm{Nb}\left(25\mathrm{nm}\right)/\mathrm{Gd}\left(d_f\right)/\mathrm{Nb}\left(25\mathrm{nm}\right)$ hybrid structures of a superconductor/ ferromagnet (S/F) type. The structural characterization of the samples, including careful determination of the layer thickness, was performed using neutron and x-ray scattering with the aid of depth-sensitive mass spectrometry. The magnetization of the samples was determined by superconducting quantum interference device magnetometry and polarized neutron reflectometry, and the presence of magnetic ordering for all samples down to the thinnest Gd(0.8 nm) layer was shown. The analysis of the neutron spin asymmetry allowed us to prove the absence of magnetically dead layers in junctions with Gd interlayer thickness larger than one monolayer. The measured dependence of the superconducting transition temperature $T_c\left(d_f\right)$ has a damped oscillatory behavior with well-defined positions of the minimum at $d_f=3$ nm and the following maximum at $d_f=4$ nm, in qualitative agreement with prior work [J. S. Jiang et al., Phys. Rev. B 54, 6119 (1996)]. We use a theoretical approach based on the Usadel equations to analyze the experimental $T_c\left(d_f\right)$ dependence. The analysis shows that the observed minimum at $d_f=3$ nm can be described by the so-called zero to $\pi$ phase transitions of highly transparent S/F interfaces with a superconducting correlation length ${\xi }_f\approx 4$ nm in Gd. This penetration length is several times higher than for strong ferromagnets like Fe, Co, and Ni, thus simplifying the preparation of S/F structures with $d_f\sim {\xi }_f$ which are of topical interest in superconducting spintronics.