The supplied chargers do, in fact, work fine with SLA batteries. Stopping at full charge, and providing a trickle after that.
HOWEVER...
All three of the chargers I have received have had their plugs (battery side) wired in reverse polarity to what the GM kit expects. If you plug the charger into the battery without correcting this, it'll smoke instantly. (Although there appears to be a reverse=polarity protection diode inside, some of the PCB traces burn up before the external fuse).
I reversed the output on the other two units, and they have worked fine, though I bought an additional charger (a 36V "Battery Minder") which I use in place of the fried unit. When the PCB traces were repaired on the latter, the charger worked fine - BUT, the end-of-charge detection circuit was gone, too, meaning it would charge forever. I tossed it.