By 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

sudo fallocate -l 2G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile
sudo swapon -s

To make this permanent add this to /etc/fstab

/swapfile none swap sw 0 0

If you need to resize the swap file

sudo swapoff /swapfile 
sudo fallocate -l 4G /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

How much swap? Well that  is down to your system. Maybe start with 1x your physical memory and then run memory diagnostics tools.