Charging lead acid batteries in series
It is normal to charge lead-acid batteries in series. As they are used, the cell voltages will change, which is why they are not charged in parallel.  If they were charged in parallel, the one with the high voltage wouldn't get much current, and the one with the low voltage would get too much current.

With the cells in series, they all get the same amount of current, and all get approximately the same amount of charge. Since they will not charge and discharge exactly the same, the battery voltage and level of charge will gradually drift apart.

To handle this, it is common to periodically do an "equalization" charge, where you overcharge the string slightly to bring up the charge of the undercharged cells. You do this because lead-acid batteries handle overcharge better than they handle undercharge.

You have done that, and at least one of the cells has gassed. Check the fluid level, and next time charge to a slightly lower voltage. Only do equalization every couple of months. If some of the cells fail, it will not be possible to charge the battery fully. When that happens, it is time to throw out the battery.