Trickle charging may not 'bring it back' as the current may be less than the battery looses naturally.
Look at the Aldi and Lidl web sites (
www.aldi.co.uk and
www.lidl.co.uk) for when they offer their small battery charger - last time it was £12.99. It is a controlled unit that will lift a dead battery to a useable point charging at 800mA, then go up to 3.8A until the battery is near fully charged, and then sink back to 800mA to keep it topped up.
If you are only trickle charging a battery it is unlikely to gas. For gas to be emitted it needs to be charged at over 2.3V per cell which a trickle charger will not do. Most modern batteries are sealed self-recombining anyway.
For the record most systems charge a battery at constant voltage, but if you want to get the capacity right up it needs to be charged at constant (low) current for a long time.