Train to Milan

Any Swiss ticket office will be happy to sell you one ticket all the way to Milan.
Don't sweat a 6 minute change in Interlaken Ost. Its normal. You should have plenty of time if you do not dawdle.

Thanks Sam! So worst case scenario, I buy one ticket straight thru to Milan but miss that Interlaken connection, would I need to buy a new ticket then from Interlaken to Milan or can I just take the next train?

That would be so if you were late because of a delay caused by the Swiss cable car, bus or train, but not if the reason is you didn't hustle. The train to Milan from either Spiez for the 6:30 departure, or Brig for the 7:00 departure, is a reserved EC train.
Are you having mobility issues? I believe you can set up assistance in advance. Too much luggage? Can't help you there, but if it is really stressing you out, just take the 6:00 cable car from Gimmelwald and you will get to Interlaken a half hour early for the first connection to Milan. Once you have a ticket you can go earlier. It is only the EC train that obligates you to make it. You could also take the 6:30 cable car intending to make the second itinerary, giving you an extra half hour in Interlaken.
Believe it or not, 5 minutes is a comfortable connection time at Interlaken Ost station. Have your luggage in hand and be standing by the door when the train stops. Its not an airplane.

Note the track numbers you need on the schedule from DB or SBB web sites. In the examples I see, you are often going from track 3 to track 2A. You will have already learned the layout of Interalaken Ost station when you arrived in that area. The connections from Gimmelwald to Interlaken run every half hour, so you could choose an earlier Gimmelwald departure than the schedule suggests, for less stress. Handling you luggage in these situations is a good reason to pack light - even more so than the idea of carrying your luggage onto an airplane, in my opinion.