Great question!

Until recently, Google Cloud didn't have reserved instances. Interestingly enough, last week they introduced their version of it called "Committed Use" pricing.

Here's what it looks like for 2 CPUs/8GB RAM (numbers below according to their documentation page):

  AWS
  On-demand = $69/month
  1-year    = $53/month
  3-year    = $47/month

  GCP
  On-demand = $52/month
  1-year    = $31/month
  3-year    = $22/month

So GCP is cheaper even considering the reserved instances!