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.

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.