[BerinG]

i guess it depends on the colour..the black stuff would probably be the sticky stuff that caught some dirt.. you can just rub it off with your thumb, just easier with something.. never had any problem with using gaz.. i wouldn't poor it on but just a little drop on the cloth.. others may be able to inform you better if the gaz causes damage but have dropped gaz under the gaz door and never did anything..

wipe it off with a dry cloth after