I haven't used Gpower in the way you're describing but I think it's a pretty self explanatory thing. I'm sure a google search would turn up a manual but it's a few drop down menus you can play with to get this. Click on the boxes that are for your study.
1)I clicked on Test family and selected F tests
2) I selected ANOVA fixed effects omnibus
3) Sensitivity compute required effect size
4) then change values in the boxes below that (power is usually set at .8)
5) then calculate
Here's a screen shot: