Thanks for the answers!

The instructions would be very complicated. Basically everyone would need HGL (incl. Patches) + HGG, extract hellgate_bghigh000.dat, hellgate_graphicshigh000.dat, hellgate000.dat, hellgate_movies000.dat and hellgate_movieslow000.dat from both (needs many GBs on HDD) and I downloaded 3 *.dat/*.idx files with more *.dds textures (they are named something like "x_hellgate_bghigh500.dat" etc.). It took hours to patch the files, because I checked every single extracted folder/file and compared them. If I just take the high .dds files from HGL and overwrite the HGG ones, then you get missing textures, maybe errors and more. As an example, the HGG GUI is different than in HGL one, some textures have the old filename, but look different in HGG, some are in different folders than in HGL... so you can't just overwrite everything and done. It will take some time and the repacking with Reanimator was a bit of a pain (I could onlay overwrite the with Reanimator extracted files, I could not add "new" files that HanbitSoft deleted) and time consuming. ^^ Not all textures are HD now, but all possible files I could find (MANY!). ^^ So new textures from HGG will still have the known resolution, because they are not available in HGL. All DX10 *.dds files (those "_dc5.dds" files) are erased by HanbitSoft by default and I don't know how to reactivate DX10, but the excel and config files for DX10 are included i HGG by default and they got edited by HanbitSoft, as far as I could see.
I will test it a bit more and start the uploading today. I tried to make a simple patch EXE with VisualPatch, but it hangs up with those big *.dat files. The HD Texture upload would be around 4.36 GB, the HiRes videos about 2,07 GB. But I will upload them free-user friendly on many hosters.
I'm not aiming any personal gain from that so far, just wanted all possible HD Textures from HGL again.

Before I merged the textures, I searched the net for some already done packages or how to do it, but there is no real download or tutorial, just speculation or the untouched HGL *.dat/*.idx files that will conflict with HGG. But I saw many threads that HD textures are wanted by many players.
BTW, where I have to post the thread for the HD textures after the upload is done?