When comparing GIMP vs Aseprite, the Slant community recommends Aseprite for most people. In the question“What are the best pixel art / sprite editors?” Aseprite is ranked 1st while GIMP is ranked 4th. The most important reason people chose Aseprite is:
You can change the playback speed of the loop and the speed of each individual frame. There are three playback modes: forward, reverse, and ping-pong. Onion Skin mode is included to speed up the animation process and to allow for tagging the timeline to help keep animations organized. There is also a live preview so you can always see the end result. Onion Skin mode will overlay previous and next frames over the canvas so you can use them as references when drawing. For Onion Skin mode, you can adjust items like range, opacity, and tint, whether the onion frames are in front of or behind the canvas, etc. You can tag different parts of the timeline when, for example, you need different animations for the same character. You can then loop those tagged sections individually.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Free, open source and cross-platform
GIMP is completely free and open source, meaning you can use GIMP and all of its features without spending a penny. This makes it an excellent case for artists or designers who may not have the budget to spend over 700 USD on Photoshop. It is also available for free on Windows, Mac OS and Linux. It's licensed under GPL with source code available here.
Pro Powerful
GIMP is the most feature rich free image editing tool. It has most image manipulation and workflow tools that you would expect from an image editing software and what it lacks it makes up with plugins.
Pro Content-aware tools with a plugin
There's a content-aware fill plugin available for GIMP.
Pro Modular & single-window interface options
By default GIMP splits out each window, but you can check to use the one screen mode in options if you prefer.
Pro Lightweight
Compared to to other photo editing software, GIMP is very light weight. In addition it isn't very resource-intensive, meaning you can put it on a flash drive and have it with you to load up on just about any computer.
Pro Beginner-friendly
Gimp has less features than programs like Photoshop, but for a beginner, or someone not needing complicated options, Gimp is the perfect choice. It allows you to quickly do basic photo manipulation so you can easily get the product you need without having to worry about complicated features getting in the way.
Pro Full channel support
Pro Good selection of tools for animation
You can change the playback speed of the loop and the speed of each individual frame. There are three playback modes: forward, reverse, and ping-pong. Onion Skin mode is included to speed up the animation process and to allow for tagging the timeline to help keep animations organized. There is also a live preview so you can always see the end result.
Onion Skin mode will overlay previous and next frames over the canvas so you can use them as references when drawing. For Onion Skin mode, you can adjust items like range, opacity, and tint, whether the onion frames are in front of or behind the canvas, etc.
You can tag different parts of the timeline when, for example, you need different animations for the same character. You can then loop those tagged sections individually.
Pro Easy to use
Getting started with the program is straightforward. It's laid out intuitively: the main workspace in the middle, color selection on the left, tool section on the right, and animation timeline at the bottom.
All tools and the vast majority of functions have keyboard shortcuts, allowing for results to be obtained quickly.
Aseprite is a very focused program: it's not filled with icons, there's no excess functionality, and dialog boxes generally only have a couple of options so you're never overwhelmed and it's easier to learn.
Pro Responsive developer
Always in touch with the community and approachable via Twitter.
Pro Awesome tools
Tools are good, easy to use.
Pro Pixel perfect mode
It makes drawing lines and shapes less jaggy by default.
Pro Free and paid versions available
Aseprite is free if you compile it yourself. Its maintainers also offer a security-signed package with a technical support license for a one time fee of $14.99. It's also easy to use and presents really awesome tools.
Pro Cross-platform
Aseprite is available on macOS, Windows and Linux. The unfree source code is published on GitHub.
Pro Made for pixel art
It was designed with pixel art in mind, unlike other general-purpose image editors. This means you get lots of useful features and very little clutter from tools and features that you won't need.
Pro Very intuitive thus easy for complete beginners
The interface invites you to be creative, and since it's pixel art you are creating, this adds to the feeling of being in the right environment. Everything seems to have its natural place and thereby could make beginners feel right at home.
Pro Beautiful interface
Aseprite has a beautiful pixel-art interface that makes it a pleasure to use.
Pro Scriptable using Lua
Aseprite is scriptable using the Lua language, which can be used to automate tasks and add new functionality. Some consider the API more user-friendly than GrafX2's.
Cons
Con Unintuitive interface
GIMP doesn't embrace OS X application design, thus Mac users might have a hard time wrapping their heads around GIMP's interface.
Con Less features than other programs
Being a free program, it shouldn't be surprising that it has limited features. For those needing more advanced features for something more professional, GIMP just isn't for you.
Con Lacks adjustment layers
Adjustment layers offer a non-destructive way of combining different photo manipulations. Without adjustment layers the only way to see changes is by irreversibly editing the image.
They are promised in future updates.
Con Poor performance
Con Not FLOSS anymore
The license was changed to a shared license, that does not allow redistribution of the source code. While older versions should still be FLOSS, the newest versions are not.
Con Text tool could be better
You can't change text and its font or size after you've inserted it. You have to re-insert text every time you wish to make an edit.
Con No tilemap support yet
A tilemap editor is on the roadmap for version 1.6.

Con Pixel-styled interface can be jarring
Aseprite uses low-resolution window frames and fonts. Opinions vary on whether this sets the mood or gets in the way.
