You normally install GWmodel from CRAN. However there are some times when this is not available. In this case use Github.

To install, we recommend the following steps are used to install from Github.

chooseCRANmirror()
install.packages("devtools",dep=TRUE)
install.packages("usethis",dep=TRUE)
install.packages("prettyunits",dep=TRUE)
library(devtools)
install_github("lbb220/GWmodel/GWmodel")