Is 1 vCPU on Google Compute Engine basically half of 1 physical CPU core?

Typically when software tells you how many cores they need they don't take hyper-threading into account. Remember, AMD didn't even have that (Hyper-Threading) until very recently. So 2 cores means 2 vCPUs. Yes, a single HT CPU core shows up as 2 CPUs to the OS, but does NOT quite perform as 2 truly independent CPU cores.

That's correct, you should aim for a GCE machine-type that has 4vCPUs... When you're migrating from an on-premises world, you're used to physical cores which have hyperthreading. In GCP, these are called vCPUs or virtual CPUs. A vCPU is equivalent to one hyperthread core. Therefore, if you have a single-core hyperthreaded CPU on premises, that would essentially be two virtual CPUs to one physical core. So always keep that in mind as oftentimes people will immediately do a test. They'll say, "I have a four-cores physical machine and I'm going to run four cores in the cloud" and ask "why their performance isn't the same?!!!"