for commercial work, GIMP is a no go. for simple personal work, be it photo correction or dabbling in design, knock yourself out, but all the design firms and departments I've worked for have all used adobe - face it, it's an industry standard, and with good reason.
not to discourage, but clothing isn't an easy task. if you're serious, do a lot of research beforehand - look into the screen-printing process (even if you're not going to actually be printing yourself, a little real world education on the subject goes a long way into understanding how to make separations, etc.), learn how to do channel separations, what the difference between raster and vector is, what resolution to work in (for screen printing, 256dpi is the magic number - anything over that is OK, but it's just overkill), how to call out colors and processes (learning what PANTONE colors are and the different types would be a good start), check out a few sites on color theory and basic design principles.
the big mistake that people make when they design is that they make something that they like without seeking a second opinion, so ultimately they put in all this time and effort into designs that in all actuality suck ass and they wonder why people aren't buying them like hotcakes - just because you slap something on a shirt doesn't necessarily mean people are going to buy it.
my advice? start designing. but with no real goal in mind. learn the basics and build a portfolio. get opinions, not just from your friends and parents, but try and find a group of people who will give you constructive criticism (NS probably isn't the best place - try taking a design class at school where everyone else is roughly on the same plane in terms of ability, you'll find the group to be much more accepting). keep building a portfolio. then after a few months or years, take a select few and mock them up on shirts, and get opinions again from different people. take into consideration the size of the piece, the color of the garment (that's a whole world in and of itself), and maybe look into different processes, such as embroidery, applique, etc.
and for a shameless plug, check out http://www.junk-apparel.com.
we've been doing clothing for several years, but it didn't happen overnight. we had to gain industry knowledge and contacts. and we're still just squeaking by.
it's not easy. if you're serious and have a drive, though, by all means go for it.
_____________________ 'yeah line stuff blows, i got a pair of the new pollards and once i took the wrapper off they spontaneously combusted' - schlonginatorjunkapparel... we make clothes.