I've looked through the Google Cloud API Gateway docs and searched the public issue tracker but haven't been able to find a mention of it one way or another.
The closest I've come is this google groups thread from 4 months ago, which suggests that it may be 60 seconds, but I haven't been able to confirm this or determine if it's out-of-date yet.
TL;DR
I'm looking for alternatives to AWS API Gateway (which has a fixed 30 second timeout) and so am trying to figure out if GCP API Gateway would work as an alternative on this particular point.