I cannot understand how to define gabions in Slope.

Two approaches can be used to define slope gabions, the first is to define them as deformable walls and assign an average shear strength in terms of tension.
The other is to use the polygon tool and then treat the polygon as a material, in which case it is suggested to use apparent geotechnical parameters:
Apparent cohesion=(0.03M(gab)-0.05)98.1 [kPa].
Apparent shear strength angle= 25*ro(gab)-10 [°].
where:
M(gab) unit mass of wire mesh (kg/m^3)
ro(gab) overall density of the gabion mesh (Mg/m^3)