Connect between two AWS regions: Software VPN Appliances

https://d0.awsstatic.com/aws-answers/AWS_Multiple_Region_Multi_VPC_Connectivity.pdf
http://cloudacademy.com/blog/openvpn-aws-vpc/
https://medium.com/connecting-the-clouds/aws-region-to-region-vpc-connectivity-with-openswan-and-vpc-vpn-c22610bdc3ad#.8qxvok1aa
http://aws.amazon.com/articles/5472675506466066
https://aws.amazon.com/marketplace/pp/B00MI40CAE/ref=srh_res_product_title?ie=UTF8&sr=0-2&qid=1466995004835
https://aws.amazon.com/marketplace/pp/B00H03HMLA/ref=srh_res_product_title?ie=UTF8&sr=0-10&qid=1466995004835

https://www.websiteops.com/cloud/2015/05/13/connecting-aws-regions-with-openswan/

AWS to GCP:
https://jumpcloud.com/engineering-blog/connect-gce-and-aws/
https://www.websiteops.com/cloud/2016/09/23/connecting-aws-to-gcp/
https://cloud.google.com/files/CloudVPNGuide-UsingCloudVPNwithAmazonWebServices.pdf