You generally can’t brighten a badly underexposed image without revealing more noise, but you can improve the result.

In GIMP, start with gentle tonal adjustment rather than extreme brightening. Community suggestions included using levels/curves, or modest brightness/contrast changes, to lift exposure while keeping the image natural. Raising shadows too aggressively will make noise and JPEG artifacts more obvious.

Then apply noise reduction. A commonly recommended option for GIMP is Wavelet Denoise, and the GMIC plugin was also suggested for denoising and general enhancement. These can reduce noise with less detail loss than heavy blur.

If the file is a JPEG, some of what you see may be JPEG blocking/artifacts, not just sensor noise, so cleanup may be limited. The TIFF may look noisier simply because it preserves more of the image data instead of hiding it with JPEG compression.

Other editors mentioned by the community for this kind of recovery were LightZone and RawTherapee, both of which offer exposure and noise-reduction tools.

Best practice: brighten moderately, reduce noise carefully, and accept that some luminance grain is often preferable to overly smoothed detail.