Be default Google Compute Engine (Debian) doesn’t come with any swap space. If you have periodic programs that eat a bit more memory (like my backups do) then instead on just adding more physical memory you can add a swap file.
Swaps files are much slower than physical memory, so should be avoided except when it is needed.
Setting up a swap file on Debian on Google Compute Engine is described here – https://geekbacon.com/2015/03/04/how-to-create-a-swap-file-on-google-compute-engine-virtual-machines/
If you need to resize the swap file see https://www.digitalocean.com/community/questions/how-to-change-swap-size-on-ubuntu-14-04
How much swap? Well that is down to your system. Maybe start with 1x your physical memory and then run memory diagnostics tools.