From GridLAB-D Wiki
The quiet option silences all but the most critical output from GridLAB-D. It is managed by the global variable quiet, which is by default FALSE.
GLM
To enable quiet mode use the directive
#set quiet=1
To disable quiet mode use the directive
#set quiet=0
Command line
To toggle quiet mode use the option
host% gridlabd --quiet
To enable quiet mode use the option
host% gridlabd -D quiet=1
To disable quiet mode use the option
host% gridlabd -D quiet=0