This is an `arcpy` solution rather than a ModelBuilder one but I believe it is pretty straightforward. To run it you just have to open the Python console in ArcMap and paste the following lines:

    import arcpy
    
    # specify the path to your GDB
    arcpy.env.workspace = 'path/name.gdb'
    
    # get all the Feature Classes in the GDB
    fcs = arcpy.ListFeatureClasses()
    
    # run the Merge tool
    arcpy.Merge_management(inputs=fcs, output='merge_result')

This will create a feature class called **merge_result** in your GDB.