Hey Alden, I'd like to thank you for using GIMP and supporting open source software and for being the first critique I've given on Deviant Art. I'll separate my points in to: Line, Colour, Perspective, Details.
It's good that you've used bold line work but I'd advise to watch where there is tagents, such as where the left breast barely overlaps with the rolled up sleeve, and the black claw along the thumb's crease. Tangents tend to break the sense of depth.
You may also want to pay closer attention to the arms, the colour breaks the lines width and either overlaps or seems to erode it. But there's a simple way to stop this happening is to take advantage of GIMP's Layer tool: Ctrl+Paint offers the best explanation I've came across: [link] (This is epsiode 4 of the 101 series)
We tend to measure and judge colour relative to what it's surrounded by. So you may want if you're showcasing characters to pick a less bright colour for the background. Pale Muted Colours don't tend to overpower a characters presence, and helps make the character stick out than the background.
A similar point on the nipple, is that they need to be less bright and more of a flesh skintone than fuschia pink, (Similar to the tone used for Anyata's fur.)
I think Is to watch where the viewer's eye level is, you have it set at different levels. If you were to cover up the body and look just at the end, is that we're eye to eye. Cover the head and lower half, and look at the chest, we're looking slightly downward (Which is good so far) but we can see under her shirt at the hips; but her posture isn't curved to hips tilting up, and we're not looking up at her.
But you also could've included that sense of wrapping around for the jacket's sleeves to give them a more three-dimensional feel. You've done this with the lapel too (Which is good) but you may also want to consider adding some depth to the pocket's flaps as well.
Some of the creases don't seem to add or don't relate well to how the clothing is wore, while some do. I mean we can see when we tug our own shirts, we create wrinkles, but there's a science to it, so to say. You might benefit looking for tutorials on Deviant Art but if you can stomach heavy reading, Burne Hogarth basically landslided this in his work "Dynamic Wrinkles and Drapery." (Which you may be able to get from a local library.)
I love military girls (I was in the ATCC myself.) and I hope you can benefit from what I've said. Yet just to re-iteriate, if you learn to use GIMPs more advanced features like layers and opacity (CTRL+Paint explains this better) it'll work as an all-in-one package, but I also avoided commenting on the anatomy because if your working purely digital, it's also really hard to do. If were looking for comments on that, I'm not sure what to say other than: Buying a pick stack of cheap-ass copier paper and drawing from life till your heARTs content.