Can you hide code cell outputs in Google Colab?

This can be done by adding --quiet at the end. Thus for your case

!pip install gwpy --quiet

I found this answer and applied it successfully: https://serverfault.com/a/41968/328943

Simply adding &> /dev/null to the tail of any command will silence its output outside of any errors that may arise.

Like so:

!pip install gwpy &> /dev/null

In this case you can just use

!pip install -q gwpy

In general, you can start the cell with %%capture

%%capture
# the rest of your code