Primero estás usando un código de PHP4, recomiendo que vayas modificando a PHP5 y segundo el "garbage collector" funciona de acuerdo a
Código PHP:
Ver original; Define the probability that the
'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
session.gc_probability = 1
session.gc_divisor = 1000
; After this number of seconds, stored data will be seen as 'garbage' and
; cleaned up by the garbage collection process.
session.gc_maxlifetime = 1440