Photoshop has non destructive image editing. That means at any point in time you can change the setting of a filter, or contrast you modified etc without having to backtrack and redo everything you did from that point. Gimp is still the stone age of image editing. Photoshop introduced the first element of non destructive editing in 4.0, 1996. The GIMP is still a worse, less productive tool than one of the oldest edition of toshop. They said they'd introduce things like these with GEGL but it's 2015 and the idea of a non-destructive workflow where everything you do can be changed and undone as a single element without touching the rest is still a pipedream for gimp.
By the way, even freewares like Picasa can handle a non-destructive workflow. Picasa is a toy, and doesn't have much in terms of feature set, but the base capability is there in the engine, something GIMP can't seem to grasp.
Krita can do it in the world of open source software, but it's focused on painting features (more like Corel Painter) rather than photo/generic image editing. If you hate the idea of using photoshop, I'd say try your luck with Krita instead. It's focused on painting tools but it still has editing tools too and you won't feel like you're using prehistoric 1980 level of workflow.
http://download.vikis.lt/doc/kde/HTML/en/krita/layers-adjustment.html
Krita actually has a competent development team and it's constantly evolving.
You can find more information on the features here:
http://www.photoshopessentials.com/photo-editing/adjustment-layers/
https://helpx.adobe.com/photoshop/using/adjustment-fill-layers.html
https://helpx.adobe.com/photoshop/using/layer-effects-styles.html
Basically, you changed the contrast, then made some other edits to your pic. You want to change the contrast again? no need to undo anything, because none of those things did permanent transformation to the picture, it's applied in real time and saved as metadata rather than transforming the actual image (that is, until you do export it as a final .png or whatever).
This kind of workflow exists since 1996. Working with anything else is like going back to the fucking stone age. Work smarter, not harder. Why waste your time on a pile of shit that takes 10x more time and effort to get things done? Sure, you can do anything with GIMP, as long as you don't give a fuck about the amount of wasted time.