As the php manual says:
session.gc_maxlifetime integer :
session.gc_maxlifetime specifies the number of seconds after which data will be seen as 'garbage' and potentially cleaned up.
Garbage collection may occur during session start (depending on
session.gc_probability and session.gc_divisor).
Note: If different scripts have different values of session.gc_maxlifetime but share the same place for storing the
session data then the script with the minimum value will be cleaning
the data. In this case, use this directive together with
session.save_path.
session.gc_probability integer:
session.gc_probability in conjunction with session.gc_divisor is used to manage probability that the gc (garbage collection) routine is
started. Defaults to 1. See session.gc_divisor for details.
session.gc_divisor integer
session.gc_divisor:
coupled with session.gc_probability defines the probability that the
gc (garbage collection) process is started on every session
initialization. The probability is calculated by using
gc_probability/gc_divisor, e.g. 1/100 means there is a 1% chance that
the GC process starts on each request. session.gc_divisor defaults to
100.
You also can refer this post.