I pull the piece of gauze off the back and put them in a pot of water and boil them. Stir them while they're boiling to scrub them together to get all the paper off. Pick the bits of paper out with a fork. When you can't see any more paper pour them out in a strainer and rinse them. Put them back in the pot with fresh water and boil again to get all the glue off. The glue will turn the water a yellowish color so boil and rinse until the water remains clear. I don't think the glue will really hurt anything but I had some algae growth on the plugs before I started cleaning them this way.
I like using these tiles because it allows me to crowd the frags closer in the frag tank. Of course if you want a peg on the bottom to keep them from moving you wouldn't like these.
Make sure to glue your frag to the back rough side, not the shiny front side. They stick better that way.
HTH