The recommended GTS spark plugs are one heat-range colder than the recommended GT spark plugs.
Colder heat-range plugs help stop pre-ignition, and are generally used in highly tuned normally aspirated and forced induction engines. For a example, the GT plug heat-range is 16, whereas the GTS heat-range is 20, 1 heat-range colder.
For a standard internals road car there is probably not much advantage to go for a colder plug, unless you drive you car in the 7-8000RPM range for extended periods.
Though, for the Celica GTS, it is recommend to use a these colder plugs because of its VVTL-I tech. A downside of cold plugs is that they foul easier than hot plugs.
Since you've put GT recommended plugs in to your GTS, the plugs you are using are 1 heat range warmer. Its recommended to keep your Celica out of high-rev ranges for extended periods -- especially on your GTS.