Use this line of code in your launch file. Set this right after <launch> at the beginning of your launch file . <env name="GSCAM_CONFIG" value="v4l2src device=/dev/video2 ! video/x-raw-rgb ! ffmpegcolorspace"/>
Or instead of setting environment variables in the launch file, edit your bash file. So first in terminal " gedit ~/.bashrc" then add this line somewhere near the bottom of the file "GSCAM_CONFIG="v4l2src device=/dev/video0 ! video/x-raw-rgb ! ffmpegcolorspace" . My web camera was set to video0. Now you should launch gscam without any problems.