Ack. Then I'm wrong on that account... and since it looks like the graphics from the GXL files did not contain ingame sprites, the ACT's are the culprit.
Opened up the ACT's in Hex Workshop. Looking at the hex structure, the files are definitely divided into a number of data entries. Their structure is not entirely clear, though there are some similarities with a sprite format I've been trying to rip not long ago.