  Two-dimensional (2D) magnets have great potentials for applications in
next-generation information devices. Since the recent experimental discovery of
intrinsic 2D magnetism in monolayer CrI$_3$ and few-layer Cr$_2$Ge$_2$Te$_6$,
intensive studies have been stimulated in pursuing more 2D magnets and
revealing their intriguing physical properties. In comparison to the magnetism
based on $3d$ electrons, $4f$ electrons can provide larger magnetic moments and
stronger spin-orbit coupling, but have been much less studied in the 2D forms.
Only in very recent years, some exciting results have been obtained in this
area. In this mini-review, we will introduce some recent progress in 2D Gd
halides from a theoretical aspect. It is noteworthy that $4f$ and $5d$ orbitals
of Gd both play key roles in these materials. For Gd$X_2$ ($X$=I, Br, Cl and F)
monolayers and related Janus monolayers, robust ferromagnetism with large
exchanges comes from the $4f^7$+$5d^1$ hybridization of Gd$^{2+}$. The
spatially expanded $5d$ electrons act as a bridge to couple localized $4f$
spins. For Gd$X_3$ monolayers, the intercalation of metal atoms can dope
electrons into Gd's $5d$ orbitals, which leads to numerous intriguing physical
properties, such as ferroelasticity, ferromagnetism, and anisotropic
conductance. In brief, Gd halides establish an effective strategy to take
advantage of $f$-electron magnetism in 2D materials.
