The weather in Gimmelwald can be extremely variable. As Gimmelwald is located in the heart of the alps and rests at 1'400 meters above sea-level, you have to be prepared for the weather changing very quickly and dramatically. Keep in mind that it doesn't get too hot in very high altitudes, even in July and August (especially at night). Snow and blizzards are always a possibility in higher altitudes, so keep an eye on the weather forecast. It is best to bring all sorts of clothes whatever the season. Most important are some good hiking boots.
The following weather reports are usually the best, here you can get a local weather report for Gimmelwald: