If I try the command gap -c Print("Hello"); on my Ubuntu, I get an error. In my case the right syntax seems to be :

gap -c "your_code"

or :

gap -c 'your_code'

Work for me : gap -c 'Print("Hello\n");' (I use ' instead of " for a correct parsing... and the \n matter !).

You can try a more simple thing : gap -c "a:=1;" and check in the console that a is indeed bound and equal to 1. However the field in GAPInfo.CommandLineOptions; for the option -c still be empty, I don't think this is where the data is stored. You can recover your input by calling GAPInfo.InitFiles;.

To sum up there is a screen of running the following command :

gap -c 'a:=1; Print("ThisOneIsDisplayed\n"); Print("ThisOneNot");'