The problem isnt the gel, it's the barrier that keeps the gel in. Water vapor has to pass out of the container for it to work. If the pores get covered with scale(lime/calcium) it will slow down or stop the evaporation.
For example.. I just replaced the water pad in my home humidifier (kind strapped to the furnace). Just going to a new pad raised the humidity 10% without touching the humidity dial.
The reason I mention this, is that the gel is cheap and easy to replace. If it was a matter of gel, you'd be better off using tap and then changing out the gel once in a while. But the gel is held in by something, either a membrane, some kinda fabric, gortex, etc. If that barrier gets scaled, you'll need to replace the whole thing.
Sponge ones usually don't have a barrier, and there's just a hole. I'm not convinced you can get enough scale on a sponge to ruin it because you wring it each time you soak the sponge. If there was scale, it would break/flake out when you wring it out.