Photoshp and the GIMP may look similar and for some things they are very similar but they are still very different programs.  Where you found some tutorials for Photoshop working quite well in PSP and some that didn't, you will find the same thing with Photoshop tutorials and the GIMP tutorials.  I mainly use Photoshop but I also use the GIMP because of its portability on a USB hard drive.  If the GIMP had adjustment layers I could almost make the switch to it.  If you are familiar with PSP and know it well try and understand what action is trying to be accomplished in the Photoshop tutorial and then figure out a PSP workaround.  Have a look at the GIMP as you have nothing to loose or spend on it but know that it will still frustrate you when you are trying to follow a tutorial written for a different program.

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