One more important thing to consider if using a
no-name or cheaper battery charger is that it may not be of the
controlled-voltage (CV) variety. What does that mean? Inexpensive
chargers will often charge at up to 16 VDC causing the battery to
overgas, leading to a rapid deterioration of battery life.

AGM (and I reckon Gel too) batteries should always be charged with a
controlled-voltage (CV) charger which regulates the level of voltage to a
safe level of about 13.2 up to 14.5 VDC leading to longer battery life.

The primary difference between these two types of chargers is that the
newer type (controlled-voltage) were specifically designed to
accommodate the charging requirements of the newer AGM style batteries. To
achieve that goal, it is necessary to modify the absorption charge
mode in the following way. The newer type of charger (CV) switch to float
mode when the charge current drops to 0.5 amps. They
switch to float mode when the charge current drops to 0.1 amps. The
result is that for an extended period of time, not to exceed 8 hours,
the CV charger's output voltage will be held at a constant
voltage that is significantly higher than the float voltage.

The switchover at 0.5 amp results in an
absorption charge mode length of approximately 1 hour. During this
1-hour period, the battery charge voltage is held constant at a value of
approximately 14.3 volts. Because of the slightly higher voltage
recharge requirements of AGM batteries, and because AGM batteries
require a longer period of constant voltage absorption, the
controlled-voltage charger controls the output voltage at 14.4 volts
while it waits for either the charge current to decrease to 0.1 amp or for the absorption
charge mode control timer to expire. The end result is that the
controlled-voltage chargers absorption period is longer and at a
slightly higher voltage than that for the cheaper or older style chargers.

I've used a Battery Tender Plus (CV) and now use a Yuasa 900
controlled-voltage charger and while I reckon regular usage of the bike with a charging system in good working order, it is always a good idea to whack a charger on even with that regular use. I will often obtain five years or more of usage
from typical FLA (flooded lead acid) and AGM (absorbed glass matte)
batteries.