The .dat extension does not refer to any particular file type, so it's impossible to predict if GIMP would handle this. If your .dat file turns out to be a misnamed image, then yes, GIMP should be able to detect the image data in it. If not, you may try opening the .dat file in a text editor: chances are, you'll see its original extension (or the source program) written in the very beginning.
In case you've received the .dat file as an e-mail attachment, expecting a .pdf, consider this Microsoft article covering TNEF issues.