Earliest time to LEAVE Gimmelwald

You should be fine with the 06:00 departure from Gimmelwald. Interlaken Ost is a fairly small station and it will probably just be a matter of walking across the platform to your next train (which will be waiting when you arrive in Interlaken). Swiss trains are very precise, so I wouldn't expect any of them to be late.

You won't be catching a Post Bus from Gimmelwald. You can't drive there and DEFINITELY no Post Buses can get there.

You'll catch the gondola (Schilthornbahn) down to the valley floor to Stechelberg where you will connect with the Post Bus to Lauterbrunnen and change to train to Interlaken Ost. A 6 minute train change is entirely easy.

So the 0600 time indicates the departure of the first gondola down to Stechelberg, to catch the bus?

Yes, according to www.sbb.ch and also according to tomorrow's schedule. There's no way to look at the date of 20 September 2017. WAY too far in the future for sbb.ch

The Schilthornbahn official schedule can be found here: http://schilthorn.ch/en/Angebot/Timetable__Tariffs/Timetable then click on the link on the right side for Timetable 2017 to download the entire daily schedule of information. It does confirm that the earliest decent from Gimmelwald is 6:00. You'll see the following connection of Stechelberg, Lauterbrunnen, and finally to Interlaken Ost.

There's no way to look at the date of 20 September 2017. WAY too far in the future for sbb.ch

Not actually true. SBB publish their timetables for a complete year. The current timetable is valid until December 2017. I just checked on SBB for 20th September, the first departure is 06:00, and it gets you to Zürich at 08:58. I hope this is early enough for your train to Munich. The transfer times quoted are all normal and no problem. That is the way Swiss Railways work.

Gimmelwald (Schilthornbahn) ab 06:00 (cable car), Stechelberg (Schilthornbahn) an 06:05
Fussweg (walk) 5 Min.
Stechelberg, Schilthornbahn ab 06:12, BUS 141, Lauterbrunnen, Bahnhof an 06:25
Fussweg 2 Min.
Lauterbrunnen ab 06:33, Interlaken Ost an 06:54
Interlaken Ost ab 07:00, Bern an 07:52
Bern ab 08:02, Zürich HB an 08:58

Thank you all, this is extremely helpful, especially the links to find the full 2017 schedules, which I definitely wouldn't have found on my own. I was losing my marbles trying to piece together each station and timing!

One last question, is it possible purchase the entire trip via a site such as SBB or bahn? Or would I need to purchase each section of trip with each respective transport provider?

Chris F is exactly correct!

is it possible purchase the entire trip via a site such as SBB or bahn

Yes, you should be able to by this as one ticket at the SBB website. Even though there are multiple companies involved, they try to make this as "seamless" as possible. But, you can only buy tickets up to 3 months in advance, so try in June!
If you have a Swiss Pass, or some other pass, you only need to buy a ticket from wherever that pass ends (German border for a Swiss pass).

I did a test on SBB for 20th April (within the time limit):

special offer Cannot be exchanged or refunded / Only valid on trains/days selected
Add-on ticket Saver fare Stechelberg, Schilthornbahn - Lauterbrunnen, Bahnhof 5.00
Saver fare Lauterbrunnen - München Hbf 43.00
total Passenger 1 48.00 CHF

So I guess the first leg ("Schilthornbahn" cable car down the mountain from Gimmelwald to Stechelberg) must be paid for locally.

The Schilthornbahn is covered 100% by the Swiss Travel Pass from the valley floor in Stechelberg all the way past Gimmelwald to Murren. Above Murren you'll get a 50% discount.

The bus picks up under canopy at the Cable car in Stechelberg.