This method will work ONLY if you are using the licenced version of DVDFab (as the freebie, DVDFab HD Decrypter currently cannot create ISO images).
In DVDFab, 'source', choose the file folder with the various 1 GB files. In 'target', make sure your DVD drive is selected, and click on the 'ISO' icon (the one beside the 'folder' icon).
DVDFab will then take those individual files and create one large ISO file on the DVD you are writing to.
After that, to get the ISO disc to work in the PS2, you'll need the PS2 boot discs and the ISO disc together in the game machine.(unless you have a mod chip installed in the game machine) and (Assuming that DVDFab copied the disk properly...) nothing I can find indicates it will, and other solutions for copying those particular games involve using VSO's Blindwrite to do the job.