Hi Kalpana -
Adding Gimmelwald makes a long day.
You need to study the timetable:
It will take you three hours to get Mürren via Gimmelwald. Use the timetable to find the shortest route. If you leave Luzern at 0705 you will be in Mürren at 1005. Trains run every hour at xx05 from luzern.. That does not include time spent in Gimmelwald on the way up from Stechelberg..
Your return route to Geneva should be most probably through Bern to save an hour or two for time to spend in the Lauterbrunnen Valley/ Mürren. Don't go through Montreux.
If you wish to go up on the Schilthorn above Mürren, you will add time to the journey.
If you leave Mürren at 1355 or 1406 (different routes) you will arrive in Geneva at 1746.
If your time is critical for arrival in Geneva, I would leave on the earlier routing, which leaves one hour earlier at 1255 or 1306.
Neither Mürren nor Gimmelwald have special things to see. You are there to walk around the villages, have lunch, look at the houses and buildings, and at the Jungfrau and mountains. From Mürren you can go up to Schilthorn.
With the small amount of time you have allowed for the Mürren area, you will not have time to walk in Bern.
The quick tour of Bern is to walk through the town to the Nydegg Bridge, then up to the Rosegarden for great views and still a few flowers, and return. You can catch a tram at the Nydegg Bridge ti front of the Brasserie Bärengraben to save time on the return.
Allow about an hour and a half. Although, you might do it faster if you walk very very fastr and do not stop or explore anything.
You are trying to fit a lot in one day. Under those circumstances, any error in catching a train, etc. can make you late to geneva. So, I would aim for an early arrival at geneva, unless you are sleeping there.