The format for Internet Gateway is:
{ "Type" : "AWS::EC2::InternetGateway","Properties" : { "Tags" : [ Resource Tag, ... ]}}
Example:
"InternetGateway" : { "Type" : "AWS::EC2::InternetGateway", "Properties" : { "Tags" : [ "Key" : "Name", "Value" : { "Ref" : "AWS::StackName"} ] } },