The last time you dumped the grey tank did flow like normal? If you think it is clogged due to a build up of food particles try a product called Unique RV Digest It. Never tried it in a gray tank however it will turn solids into almost clear looking in the black tank - you do have to leave it set for up to 24 hours for this action to take place.
Everett & Joan after 60 years together. 2004 1/2 ton Chevy & 2011 Rockwood 1809S
Number of nights camped in 2012 65
Number of nights camped in 2013 82
Number of nights camped in 2014 105!
Number of nights camped in 2015 81