Is there any way to adapt the levels of an image through a specific 'level' layer, like in Photoshop?
I find it very handy to have it as a layer.
See below, I haven't found a way to achieve this.
Although there are no adjustment layers in GIMP as xenoid has already said, you could create a new layer from visible using Layer > New from Visible, and then apply levels to that new layer. At least that way the original layers are not altered and are still available if you need to redo the levels adjustment.
Need adjustment layers, but must stick with freeware? You have some luck. Krita has them. There the name is "Filter layer". In addition you can insert readjustable adjustment masks which affect only a single layer, not everything below.
Actually the system design in Krita is here and there more coherent than in Photoshop. For example you need smart objects and -filters in PS to have readjustable blur. In Krita it's a Filter layer or a mask just like levels. Unfortunately all filterings do not work as filter layers.
ADD 3 years later
In 2022 online image editor Photopea has reached so much of Photoshop's functionality that it's well worth checking. There's available adjustment layers, layer styles, smart objects and smart filters. Only RGB is supported, but the program is still remarkable.
It's still free, but shows ads. Photopea has started to want a payment if one wants to shut down ads.
GEGL (Generic Graphics Library) is a data flow based image processing framework, providing floating point processing and non-destructive image processing capabilities to GNU Image Manipulation Program and other projects (imgflo, GNOME Photos, iconographer, …)
With GEGL you chain together processing operations to represent the desired image processing pipeline. GEGL provides operations for image loading and storing, color adjustments, GIMPs artistic filters and more forms of image processing GEGL can be used on the command-line with the same syntax that can be used for creating processing flows interactively with text from GIMP using gegl-graph.