I used GIMP many years ago - but it's been ages. At that time it's filetypes were extremely limited and restricted to 8-bit only (I was surprised to read Sparky's comment because I thought they supported 16-bit images and frankly I thought they could import RAW by now.)
Anyway, I quickly realized that to speed along the adjustment process/workflow and also to manage the images that I'd be shooting for years, that I'd want software built for such a purpose and switched to Aperture on the Mac, which fulfilled about 98% of my needs and Photoshop covered the other 2%. Ultimately Apple decided to discontinue Aperture development so I realized that if I wanted ongoing support I'd probably have to switch to Lightroom (which I've done now.)
I really prefer Lightroom for adjustment of most images and it's purpose built for this more so than Photoshop.
On a budget, however, you really should look at Photoshop Elements and there are several reasons for this:
#1 - you can actually "buy" Photoshop elements and it's currently $70 (normally $100). You don't have to "rent" it (the only way to get Photoshop CC is to "rent" it via the Adobe monthly subscription model, but while they call it a "monthly" subscription it's really an "annual" subscription because you cannot rent it for less than a year (they'll let you pay for it by the month.)
#2 - depending on how much Photography you do, you may eventually want to get the full-blown version of Photoshop. The user interface for Elements is basically the same (Photoshop does more... but it looks the same). Both have a learning curve, but once you learn to use Elements, you'll feel "at home" when you start using the full-blown version.
#3 - there are TONS of books and online tutorials... TONS... there is no other photo editor for which you can find nearly so much help online.
#4 - you probably don't need the full-blown version unless your hard core. Most of the editing features that a photographer would need or use are found in Elements. There are features of the full-blown version of Photoshop... such as CMYK color separations that a publishing company relying on offset 4-color printers (something no photographer would own or use). The vast majority of what you'd need is found in Elements. But again...if you ever needed to take something to Photoshop, it would recognize your Elements files.
With all of this aside... again, I use Lightroom for 98% of what I do. It's much much easier. You mentioned GIMP was not very user-friendly (it has a learning curve) but frankly Photoshop isn't very user-friendly either. Lightroom is vastly more user-friendly. It's technically still possible to "buy" Lightroom 6 (Adobe makes it very difficult to find the link, but if you want it, we can provide you with a direct link). It's about $150. The other way to get it is to subscribe to the Adobe Creative Cloud bundle for photography $10/month with a minimum 1 year subscription commitment and it includes both Lightroom CC and Photoshop CC (it also includes Lightroom Mobile if you have an iPad and a couple other mobile apps for Creative Cloud.)