[bojan]

Just an adequate series resistor with 12V power supply will do..
The good charging current for your lead gel batteries is around 10% of capacity, that is 400mA.
So, put 1.5 ohm/2W in series with 12V power supply adapter (for one battery). Charging time is 10 hours or a bit more but not much more than that.

Another way to do it is to add a small charging circuit (actually a series voltage regulator), with output adjusted to 13.6V. I can send you a schematic if you go this way, it is just a transistor (mounted on heat sink) and couple of components (zener, resistors, capacitor or two).