My gw2.dat file is about 38 gigs in size with both exapansions installed. Normal? Any use in deleting it and let it rebuild?
It's normal. Also, it doesn't matter what expansions you own, it's all downloaded. If you delete Gw2.dat, it should end up being a little smaller since it won't include stuff that is no longer being used.
Why you wanna delete it? It's not that data is expensive these days.
My up-to-date GW2.dat file is 36.9 GB and was re-downloaded 5 months ago or so. If you really need the disk space, you can delete the file and let it download all over again, but be prepared to be downloading for some time. If you don't need the disk space, don't bother redownloading.
If you have a HDD, I would recommend to cleanup and defragment it after you deleted GW2.dat. This allows the newly downloaded file to be stored in one contingent place on your HDD. When you have a SSD, defragmenting is not needed.
Now a followup question - my guess is that gw2.dat is actually made up of several files (kind of like Quake .pak files) compressed into one, big file. If so, is it possible to extract and run the game with extracted files? I think that would be nice in combination with my rather small ssd cache disk.
GW2 packs everything into Gw2.dat, which contains around 400k files. Is it possible to play with extracted files? In short, no, it's not something that GW2 supports. It is however possible if you wanted to put a lot of effort into it.
If you just want to minimize the size of Gw2.dat, only let it download up to the playable state and set content streaming to on demand in the options.