There should be no problem connecting two WS-G5486 GBICs across 6km of Belden M9W384 SMF cable. IEEE 802.3z spec is 5km but most manufacturer's GBICs will work to 10km.
Per Cisco data sheet: CISCO 1000BASE-LX/LH GBIC
The Cisco 1000BASE-LX/LH GBIC (WS-G5486) fully complies with the IEEE 802.3z 1000BASE-LX standard. However, its higher optical quality allows it to reach 6.2 miles (10 kilometers) over single-mode fiber (SMF), compared with the 3.1 miles (5 km) specified in the standard.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...