https://www.pveducation.org/pvcdrom/lead-acid-batteries/operation-of-lead-acid-batteries

Voltage of lead acid battery upon charging.
The charging reaction converts the lead sulfate at the negative electrode to lead. At the positive terminal the reaction converts the lead-to-lead oxide. As a by-product of this reaction, hydrogen is evolved. During the first part of the charging cycle, the conversion of lead sulfate to lead and lead oxide is the dominant reaction. However, as charging proceeds and most of the lead sulfate is converted to either lead or lead dioxide, the charging current electrolyzes the water from the electrolyte and both hydrogen and oxygen gas are evolved, a process known as the "gassing" of the battery. If current is being provided to the battery faster than lead sulfate can be converted, then gassing begins before all the lead sulfate is converted, that is, before the battery is fully charged.
Disagree with the above statement…a lead-acid battery can only convert electrical energy back to chemical energy at a set rate. If more current is applied, the excess current introduces heat into the battery. This increase of internal heat will accelerate the corrosion rate of the positive plates and reduce the life of the battery. We set limits to max. charge current and max. charge voltage to keep the internal temperature at an acceptable temperature as to not reduce battery life.

Gassing introduces several problems into a lead acid battery. Not only does the gassing of the battery raise safety concerns, due to the explosive nature of the hydrogen produced, but gassing also reduces the water in the battery, which must be manually replaced, introducing a maintenance component into the system.
 I agree with this statement as pertaining to flooded batteries.

In addition, gassing may cause the shedding of active material from the electrolyte –
 disagree…the shedding is lead from the positive grids and is caused by the natural chemical reaction of sulfuric acid and lead. However, by adding certain elements to the grid alloy, in our case, Tin, reduces the corrosion rate which increases battery life.

– thereby permanently reducing battery capacity.
 agree
For these reasons, the battery should not regularly be charged above the voltage which causes gassing. 
disagree…for a flooded battery, the gassing mixes the electrolyte when the batteries are topped off with water.

The gassing voltage rate changes with the charge rate.
 A higher charge voltage will cause the battery to produce more gas. When sizing a ventilation system, the rate of hydrogen produced used should be at the highest charge voltage the battery could be charged at.
VRLA (Valve Regulated Lead Acid) batteries, will produce hydrogen & oxygen the same as a flooded lead-acid battery. The main difference is a VRLA battery will recombine 95% of the hydrogen & oxygen back into water. Hence VRLA batteries are not required to add water as a flooded lead-acid battery.