mapply basics? - how to create a matrix from two vectors and a function

Question

I am trying create a data.frame from which to create a graph. I have a function and two vectors that I want to use as the two inputs. This is a bit simplified, but basically all I have is:

relGPA <- seq(-1.5,1.5,.2)
avgGPA <- c(-2,0,2)

f <- function(relGPA, avgGPA) 1/(1+exp(sum(relGPA*pred.model$coef[1],avgGPA*pred.model$coef[2])))

and all I want is a data.frame with 3 columns for the avgGPA values, and 16 rows for the relGPA values with the resulting values in the cells.

I apologize for how basic this is, but I assure you I have tried to make this happen without your assistance. I have tried following the examples on the sapply and mapply man pages, but I'm just a little too new to R to see what I'm trying to do.

Thanks!