An Internet Gateway (IGW) is a logical connection between an Amazon VPC and the Internet. It is not a physical device. Only one can be associated with each VPC. It does not limit the bandwidth of Internet connectivity. (The only limitation on bandwidth is the size of the Amazon EC2 instance, and it applies to all traffic — internal to the VPC and out to the Internet.)
If a VPC does not have an Internet Gateway, then the resources in the VPC cannot be accessed from the Internet (unless the traffic flows via a corporate network and VPN/Direct Connect).