I don't know beans about guage right now. All I can tell you is what I did to start mine:
I'm using plastic needles, size 10.5. The yarn I'm using is Lion Brand Kool Wool black lot 0090 color 153.
I casted on 55 stitches, but you might want to do 45 unless you've come up with a pair of longer 10.5 needles. Once I started garter stitch on them, the yarn started bunching on the needles because it was too wide.
Here's the gauge information from the yarn label:
Knit: 12 sts + 18 rows=4" in St st with size 10.5 needles. I haven't done a gauge swatch yet.
Also, I did about 10 or 12 (lost count) rows of garter before I switched to stockinette, because I liked the look of the chunky border.
I'm not really going to count the rows. I figure once I can fit my longest needle in, I can do my other garter border and bind off.
By too dense, I think she means if your stitches are too small or tight, you won't be able to poke your needles in!
Hope some of this helps you!