app.controller('gameplanController', ['currentAuth', '$scope', '$location', '$firebaseObject', '$firebaseArray', '$http', function (currentAuth, $scope, $location, $firebaseObject, $firebaseArray, $http) {

    var ref = firebase.database().ref("matches");
    var matchList = $firebaseObject(ref);

    matchList.$loaded().then(function (data) {

        $scope.checkIfDataExists = function () {
            var matchList = [];

            if(matchList.length === 0) {
                console.log("empty");
                $scope.checkIfDataExists = true
            } else {
                console.log("full");
                $scope.checkIfDataExists = false
            }

        };

    });

}]);

<button type="button" class="btn btn-default button-main" ng-disabled="checkIfDataExists">{{ gametable }}</button>