Currently, there are no virtual images available and there are no plans to have them. Sorry.
However, Teltonika devices run RutOS operating system, which is based on OpenWRT, a Linux OS distribution. Hence, even though RutOS is different from OpenWRT, they still share many similarities. It should be possible to run OpenWRT on a VM and GNS3. You can take a look at the OpenWRT documentation here and here, as well as GNS3 website here.