Very bamf tutorial. I followed along in GIMP to figure out how to do it and to translate the tools you use into their GIMP counterparts for you.
So, in GIMP:
New Layer: Layer > New Layer (Shift + CTRL + N - the same shortcut as photoshop). You can make sure the layers window is showing (I don't think it does by default in GIMP) by going to Windows > Dockable Dialogs > Layers.
Changing color: click where the green star is.
You can change the brush under where the color is (green star) that's being covered up by 'change foreground color'.
On the layer box there's some text and shit above where the layers are. One has a drop-down box and is called 'mode'. Then go to color.
To clean it up, you can use the eraser tool (shift + E in GIMP and just E in photoshop).
A trick I use for deleting shit in one layer that appears in another is to click on the other layer and use the magic wand tool (W), called the fuzzy select tool in GIMP). Select something (like the background of the image above). Then click shift and click more stuff that's part of it (so it selects multiple things). Then after you're done selecting things in that layer, leave them and switch back to the layer you were editing. Click 'delete' on your keyboard (under insert and by end). It doesn't always work but it certainly saves time when it does. That probably made no sense without visuals. FAIL.
Of course 'undo' is helpful (CTRL+z) undoing actions in both photoshop and GIMP.
Opacity is under 'mode' on the layers box. Adjust the bar or type in text as in photoshop.
Filter > Blur > Glussian Blur is the same as photoshop.
To crop I don't think it lets you do a perfect square. So, click crop and below it (where brushes were before) check fixed and make sure the size is 100px x 100px. That should do the same. Then go to Image > Scale Image > 100px x 100px.
Hue/Saturation and Levels in GIMP are under 'Colors'.