The simplest way to install the package is using devtools :
devtools::install_github("VincentGodard/gtbox")
Some of the functions in this package will use the rgrass7 package to connect to GRASS GIS, which must be installed on your system. These functions will need the directory path to GRASS binaries and libraries, containing bin and lib subdirectories (gisBase argument). On a Linux system this can determined by running echo $GISBASE inside a GRASS session.
You must also install a few GRASS addons, which can be done easily in a GRASS session through the module g.extension :
r.stream.basinsr.stream.distancer.stream.order