複数のlayerのプロパティを一括で選択する方法

If you want to select Transform/position from multiple layers, select the layers, press P, then drag a selection around all of the position properties in the timeline and enter new values. All layers will be updated.

If you have added Gaussian Blur to multiple layers and you want to adjust all of the blur values type the first few letters of Gaussian in the Search field in the Timeline, then a comma, then Blurriness and all layers that have Gaussian Blur applied will show up with the Blurriness property ready to animate. You can Ctrl/Cmnd + select all of the properties in the timeline then type in a single new value.