As @JoshObrien says, gIntersection(...)
is the way to go.
library(sp)
library(ggplot2)
library(rgdal)
library(rgeos)
setwd("<directory with shapefiles...>")
greg <- readOGR(dsn=".", layer="GREG", verbose = TRUE, stringsAsFactors = FALSE)
borders <- readOGR(dsn=".", layer="GEO_adm0", verbose = TRUE, stringsAsFactors = FALSE)
ossetes <- greg[greg$G1ID == 849,]
ossetes.df <- fortify(ossetes)
georgia.df <- fortify(borders)
intersect <- gIntersection(borders,ossetes)
intersect.df <- fortify(intersect)
ggplot()+
geom_path(data=intersect.df, aes(x=long,y=lat,group=group), colour="blue")+
geom_path(data=georgia.df, aes(x=long,y=lat,group=group), colour="red")+
coord_fixed()
BTW: In future, please provide links to your shapefiles. GREG Georgia