Which AWS managed iam policy should be used for allowing transit gateway full access?

Iam creating a role and trying to attach an was managed policy for transit gateway full access. But I am not able to find any policy with transit gateway.

There is no such AWS managed policy. So you can create your own customer managed policy. For example:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "FullTransitGatewayPermissions",
            "Effect": "Allow",
            "Action": [
                "ec2:*TransitGateway*"
            ],
            "Resource": "*"
        }
    ]
}

Depending on exactly what you need, you can add more permissions or be more selective.