If you were to, say, use Google, the first hit on
GATK parallelization is this page that literally has the phrase:
There are two options for multi-threading with the GATK, controlled by the arguments -nt and -nct, respectively, which can be combined
Please invest some effort before you ask people for help. It helps your credibility when we can see that you've tried your best. The current situation makes you look lazy.