GIMP is often referred to as the poor-mans Photoshop. Is this a fair assessment? Perhaps. Users tend to be split into three groups: those who can afford Photoshop, those who cannot, and those who are willing to pirate. For those who cannot afford it and aren't willing to pirate (we're certainly not encouraging it), then the most attractive option is GIMP.
Is GIMP an adequate alternative, however? Is it worth the effort to learn the new menu system and swap out the monster-sized commercial program for the lighter, free, open source graphics program? For most users, the answer is yes.
This tutorial will teach you how to configure GIMP to make it as close to Photoshop as possible, accomplished via plugins, filters, and other downloads. The modifications done in this article are performed on the regular GIMP, not GIMPshop. If you are absolutely unwilling to learn new keystrokes and menu layouts, then you may want to look at GIMPshop, which is a redesigned GIMP layout for Photoshoppers. If you don't like GIMPshop, you can also try GimPhoto.
How to Install Brushes, Filters, and and Plugins in GIMP
To add brushes, filters, and plugins to GIMP, simply open the program folder and put the files into their appropriate folders--brushes into Brushes, filters into Filters, etc. In Mac, right click and open the app folder. In Linux, navigate to your Home Folder and press Ctrl+H. In Windows, it's in C:/Program Files/Gimp.
Arrange the Layout
GIMP has compromised and changed the layout to be the best of both worlds. If you enjoy the free floating windows so common in GIMP, those features are still available. If, however, you want everything to be contained in one window, simply maximize the menu window, and then place the toolbar and window dock inside. Right click and choose "Always on top". It will now function just the same as Photoshop's all-in-one window; the individual docks aren't all listed in the bottom, they are all contained within the one window.
Add Photoshop Plugin Support
If you've invested in a library of Photoshop plugins, then the last thing you'll want to do is leave them behind when you switch. Luckily, it is possible to add support for Photoshop plugins using the PSPI plugin.
Download the PSPI plugin for your operating system and copy the file(s) to your GIMP plugins folder. The menu feature can be found in Filters > Photoshop Plug-in.
Higher Bit Support
Navigate to Colors > Use GEGL and check the box next to it.
Add CMYK Color Separation - http://cue.yellowmagic.info/softwares/separate.html
To add CMYK separation, you will need to add the Separation+ plugin. Download and copy it into the GIMP plugin folder.
Add Layer Styles
Can't live without Drop Shadow, Inner Glow, etc? Then install this plugin. Simply add the script to your GIMP script folder, then close and reopen the program. You'll find the Layer Styles in Script-Fu > Layer Effects.
Add Brushes
While you're adding everything to your install, download a bunch of brushes and add them to your brush folder. The default brushes that come with GIMP aren't all that great. A good resource for brushes is Blendfu.
Conclusion
The above will add a few extra features to GIMP that increases it's functionality. It doesn't have all the features of Photoshop, many of them due to copyright issues. For the vast majority of users, however, GIMP is a very good graphics program that will do everything you need.
Source: http://laptoplogic.com/resources/configuring-gimp-2.6-to-replace-adobe-photoshop
