Ok, so you've got selected_item bound to ng-model and it is an object with a property of Tags that points to an array. Instead of trying interpolation on the value attribute of the input, make the input use ng-model and update the value when the selected item changes with ng-change. This is the more "angular way" of doing it.
<select ng-model="selected_item" name="select" id="gallery_list" ng-options="v as v.Title for (k, v) in Items" ng-change="updateTags()"></select>
<input type="text" name="Tags" ng-model="selectedTags" placeholder="Tag, Tag 2"/>
//in the controller
$scope.selectedTags = null;
$scope. updateTags = function(){
$scope.selectedTags = $scope.selected_item.Tags.join(); //join() creates a comma separated string of values from the array