It really depends on what you are actually using, so it could potentially be a whole lot, or very little.
Non-Destructive Editing: There is no real non-destructive workflow implementation in gimp. The only thing resembling this is layer blend modes for dodging and burning, hue and saturation, but with smart objects and adjustment layers, photoshop is much better equipped. With lightroom and camera raw you could potentially keep your entire workflow based on the raw file. The same goes for transformations. If you scale or rotate a layer in photoshop, you have the option to scale it up again and image quality is maintained on smart objects.
Image quality: This is a small one, but purely from a technical standpoint the quality of many function in photoshop like scaling, rotating, warping, sharpening, noise reduction, etc. is much higher than gimp.
Sharpening and Noise Rediction: Lightroom has one of the best sharpening and noise reduction algorithms for raw files I know of. Neither Gimp nor Darktable come close to that. Lightroom causes very few artifacts if any and reduces noise gradually to maintain a natural look
Color control: Both lightroom and photoshop have very good tools to manipulate color - hsl, color balance, curves, blend ranges, etc. - Gimp is not only lacking many tools, the quality of its tools is overall lower too, for example masking for its selective color adjustments is less natural.
Selection tools: Gimp has no automatic selection tools other than its very old wand tool. I would guess Photoshop has about 10 or perhaps more separate functions and tools geared towards automatic selections.