Undergravel filters are pretty much going the way that 8 track tapes & slide rulers went. They still work, but there are much better filtration systems out there. Run a tank with ugf for a year, then tear it down, which is what is recommended for ugf systems. It looks like a mud puddle under those plates, real nasty.
Unless they died of old age, there is a reason the gouramis died. Water tests should have been done after the deaths, this would help rule out or confirm some things.
If you want to upgrade filtration, look into Aqua Clear filters. I have these running on most of my tanks, and have run them next to the Whispers. The Whispers choke while the ac's keep running. If the Whispers were a better filter, I would run them.
Best filtration setup I have yet to beat is an ac along with an Eheim canister. If you don't mind spending a few bucks on the canister I would go that route.