Gimp doesn't have an Exposure setting like that, which is kind of a lacking point, but on the other hand easily worked-around by using the curves tool instead. It appears that recently, the trend in Gimp has been to focus the software as more a graphics design and image manipulation tool rather than photo post-processing software. That's a bit too bad for us photographers, but the fact is that such adjustments are really better done earlier in the workflow, ideally by getting the exposure right initially — or, failing that, in RAW development.

Anyway, you can see what the Gimp "Brightness" adjustment is doing by using the Brightness/Contrast control and setting the value to +100 brightness, then hitting "Edit as Levels" and in that tool hitting "Edit as Curves". You'll get a Curves dialog similar to this:

The gray line from corner to corner is the existing file, and the black line is the new mapping.

This is, unfortunately, exactly the opposite of what you want to do to emulate the (non-motion related, of course — we'll take a static scene as a given!) effect of a longer exposure. What you want, instead, is a mapping like this, which is effectively multiplying the brightness of each part of the image by a constant factor:

Unfortunately, this interface doesn't give you anything to easily see numerically the amount of increase (let alone in a photography-related idiom like "stops" measured in EV). But, really, what you probably want is something like this:

which brings up the shadows a little bit, the midtones even more, and then has a rolloff preserving the highlights instead of clipping them.

Or, you may want this modification of the classic contrast-enhancing "S" curve, which is shifted a bit to add contrast while also pulling up the midtones:

Try these on your own image with "preview" enabled and see what they do. It's unfortunate that there's no easy, EV-based dialog, but overall, the enhanced flexibility is arguably better anyway (in Photoshop too!).

Or (especially when exposure is way off), use Darktable or Rawtherapee or (distant third in the UI department but technically fine) Ufraw before importanting to Gimp.