You also need to put some data in the data frame. I did this in the example below, though I do not know what your data look like and I just invented something. As you can see, I got a plot. Try making data that are similar to your data and see what happens. You can post the result here if you get stuck.
library(ggplot2)
data.frame(PlateLocSide = c("IN", "IN", "OUT", "OUT"),
PlateLocHeight = c(1,2,1,2),
ExitSpeed = c(90, 80, 100, 110)) -> GameData
ggplot(GameData, aes(x=PlateLocSide, y=PlateLocHeight)) +
geom_tile(color = "white", aes(fill = ExitSpeed)) +
scale_fill_gradient(low = "blue", high = "red")

Created on 2019-12-25 by the reprex package (v0.2.1)