So, if Garmisch-Munich and Munich-Salzburg are on different days, using the Werdenfels-Ticket one day and the Guten-Tag-Ticket another day works.
Depending on where you are staying in Munich, you might want to use a Bayern-Ticket, which includes local transportation in Munich, instead of the Guten-Tag-Ticket.
The Werdenfels-Ticket is an offering of the local Bahn affiliate and is valid on all regional Bahn trains and also on privatized trains run as regional trains of the Bahn. The S-Bahn in Munich is a regional train of the Bahn and is included in the Werdenfels-Ticket.
Meridian is a private company running trains under the Bahn privatization policy. The Guten-Tag-Ticket is a private offering of Meridian and is only valid on Meridian trains, which run Munich-Rosenheim-Salzburg, Munich-Rosenheim-Kufstein, and Munich-Holzkirchen-Rosenheim. It is not valid on other routes, such as Munich-Mühldorf-Freilassing-Salzburg, or on non-Meridian trains, such as the S-Bahn in Munich.
The Guten-Tag-Ticket is also recognized on the trains of the Bayerische Oberlandbahn (BOB) south of Munich.