You have saved it as an object, in this case gl. The assignment operator <- returns "invisibly" so nothing is printed, e.g.
x <- 4
Does not print 4 in the console.
To see the plot, you need to "print" it, adding a line with only gl in the code chunk or surrounding the entire operation in parentheses () will do the trick, e.g.,
```{r}
x <- 4
x
```
or
```{r}
(x <- 4)
```