swiss pass

I would suggest that it wouldn't save you money as most of the transportation you will be on will be the regional type trains. You will be on high-speeds from Milan to Spiez and from Geneva back to Milan but your Swiss pass will not cover the train ride south of the Swiss border. True the local transportation fares in Switzerland are a bit expensive, a pass for two days may not say you money. Note that when you try to buy a ticket to Gimmelwald in Italy, the Italian train system will not see it on their screens. They can't see into the Lauterbrunnen Valley on their screens. Just buy your ticket to Interlaken Ost and then buy your ticket to Gimmelwald when you get there.

Have a look at the EXCELLENT railpass information on this site. Depending on what else you want to do, a 3 day Flexi Pass may be a good deal. As the name implies, it gives you three days of travel within a 30 day period, and in between your first and last days it acts as a Half Fare Card. However, if you are planning to do a lot of moving around in the Lauterbrunnen valley a full multi-day pass could work out since you also need to consider the cost of the cablecar to and from Gimmelwald every time. The only way to decide is to plan out all your expected travel and do the math for each option.

Transport in Switzerland can be expensive, and some sort of Swiss rail pass is often a good idea. At least have a look at the Swiss Half Fare Card which discounts all Swiss transport by 50%.