I'm still too unfamiliar with async functions to do what you said but I was able to get what I wanted using boto3 like so:

apigwclient = boto3.client('apigateway')
existingapigws = apigwclient.get_rest_apis().get('items')

for gw in existingapigws:
  name = gw.get('name')
  if name in names:
    names.remove(name)

So I'm just removing gateways that already exist from my list of ones to create before the main loop