Hi,
For example, there are 5 makers on the map.
Each of them has a description.
Click on any marker to display the description.
How can I show all of them when the map is loaded?
There is no option in the Custom View Tag to show all description of the markers while the map is loaded.
Check the API from the google map documentation and check if there is any options/method to change it. If yes, you can subscribe the "map" event in Client Script and update the options or arguments.
For example:
$(document).on("map", function(e, args){
// code to update the arguments.
}