I have a project where the video on one of the video tracks is too load. Can I simply reduce the volume of that track, rather than clicking on each video snippet on that track and reducing its volume?

There only is audio on 2 tracks: That video track and an audio track. The volume of the audio on the audio track is just fine.

I figured out how I can select all clips on a track. However, when I select the and add the "gain" effect to reduce the volume, only the volume of the clip I right-clicked on to add that effect is affected by it. All the other clips stay at the original volume. This is even the case if I first group all of those clips together.

I figured it out. You can not only add effects by right-clicking on snippets but also by drag and drop effects from the effects list. You can actually drop an effect on the entire track. For this, drop it onto the track's name.