GTM Server Communication

I'm setting up LTM and GTM in a lab environment for a customer to test IPv6, on a short timeline. LTM is working fine, but I'm starting on the GTM, and cannot get the Server configuration to show as active. I can ping the LTM from the GTM, so routing apparently works, but for some reason, the server shows a blue square icon (unknown, enabled and checking). I've configured the Server using both hostname and its IPv6 address, but since this is a lab environment, we're really not doing certificates. I can't find anything that indicates if the certificate issue is a potential problem or not. I've configured two health monitors (bigip and gateway_icmp) but specified only one needs to succeed. Still, no apparent success in connecting with the LTM. I didn't think there was anything that was needed on the LTM to create this connection, but I'm running out of ideas. I have NOT configured a prober pool yet, so that may be the next thing to try. Anyway, let me know if there are any suggestions about what might be wrong, or information on how to troubleshoot this.