From GridLAB-D Wiki
The --threadcount command line option is used to indicate how many threads are allocated to the GridLAB-D run. The threadcount global variable is set using this option. The default thread count is 1.
GLM
#set threadcount=4
Command line
host% gridlabd --threadcount 4
or
host% gridlabd -T 4
Note
TECHNICAL MANUAL
The use of multithreading and the process table are not compatible with each other. The process table assume that one processor is assigned to one simulation, which is impossible when the thread count is not 1.