The -lg/loadGrammar flag does not work from the command line (works fine with the GUI). However, there is a way to import grammars using the following trick. Before the genr8 command, use the following command:
optionVar -sv loadGrammar C:BackslashProgram.FilesBackslashAliasWavefrontBackslashMayaFourDotFiveBackslashbinBackslashplugMinusinsBackslashFILENAMEDottxt
This defines a variable in the scene that will be read by Genr8 when it starts up. The part after loadGrammar is the path to the file. For some odd reason, Genr8 encodes dots, dashes, spaces and backslashes in a special way. Hopefully, the example explains this adequately. FILENAME is the name of the file.