The .gdb file (file geodatabase) cannot be directly converted to a .mdf file (SQL Server database file). Instead, create a new geodatabase on the database server, then import or copy and paste the data from the file geodatabase to the geodatabase on the database server.
File geodatabase is an Esri storage format. It has nothing to do with Microsoft's SQL-Server
database structure. They are incompatible, in the same way that diesel fuel can't be used
by a gasoline engine.