Assuming you have Gimp installed, right click on the image you want to open (as it appears in the finder, not preview) and hover above 'open with'. If Gimp is there, select it, if not, hit 'other' and navigate your way to Gimp.
Alternatively, if Gimp is in your dock, drag the target image from the finder to the Gimp icon in the dock.
Edit: I misunderstood. The only place to use Gimp's tools is, of course, in Gimp.
"Do not abandon yourselves to despair. We are the Easter people and hallelujah is our song."