Grindelwald

You don't "book" a gondola. it is a continuously moving cable with a detachable cabin about every 30 feet.
https://en.wikipedia.org/wiki/Gondola_lift

It takes 38 minutes to go from Lauterbrunnen to Grindelwald using the train that goes toward Interlaken, you get off at Zweiluetschinen and change to the train going to Grindelwald.

Grindelwald is our favorite place in the Berner Oberland, breathtaking but with more services than Murren and Wengen. We didn’t care for Lauterbrunnen as it is down deep in a valley, felt dark, whereas the others are up high in the mountains. We had a car so drove everywhere but there are trains to check out.
I would wait to buy tickets as so much is weather dependent.

Okay, I edited all my previous comments out. I think I now know what you’re referring to. Take the cog train up to Wengen. Take the tram from Wengen up to Mannlichen. Then take the next tram down to Grindelwald. Otherwise take the train from Lauterbrunnen to Grindelwald. Maybe someone else can chime in on whether they take reservations on the trams during the summer other than the Jungfraujoch, I doubt it.

No, don’t book anything ahead, just right before you get on the train/cable car.

Train/quickest option - Lauterbrunnen > Zweilutschinen > Grindelwald

“Scenic” route - Lauterbrunnen > Wengen > Cable car to Mannlichen > Beautiful Summer walk to Kleine Scheidegg > Train to Grindelwald
OR
Lauterbrunnen > Wengen > Cable car to Mannlichen > Cable car down the other side of Mannlichen to Grindelwald
OR
Longer all train route but also scenic: Lauterbrunnen > Kleine Scheidegg > Grindelwald (sit on the right side from Wengen to KS)

If you have Berner Oberland Passes, the Lauterbrunnen—-Wengen—-Männlichen route to Grindelwald is fun and very scenic. If you don’t have the BO pass it is expensive.

Take the tram from Wengen up to Mannlichen. Then take the next tram
down to Grindelwald.

We do not have trams here. Trams you find in cities like Bern and Zürich :-)