ggcyto makes ggplot2 to be able to work with Cytometry data, namely flowSet/ncdfFlowSet or flowFrame S4 objects.

library(ggcyto)
data(GvHD)
fs <- GvHD[subset(pData(GvHD), Patient %in%5:7 & Visit %in% c(5:6))[["name"]]]
fr <- fs[[1]]

p1 + geom_hvline(data = rectangleGate("SSC-H" = c(550, Inf)), colour = "red") #hline