The short explanation
Two numbers describe the same thing from different angles. MHz is the clock rate — 600MHz for the older HDMI generation, 1500MHz for HDMI 2.1. Gbps is the resulting data rate: 18Gbps and 48Gbps respectively.
What a format costs is roughly pixels × frames × bits × chroma factor. That is why 4K60 4:4:4 8-bit fills an 18Gbps link, and why 4K120 needs the 48Gbps generation.
Where you run into it
The link runs at the speed of its weakest element. Source, every device in between, every cable and the display — the lowest ceiling wins, and nothing announces which one it was.
When a mode fails, the useful question is not “is my TV 8K” but “what did this link actually negotiate”. The FRL rate answers that directly.
How the DR HDMI 8K handles it
The DR HDMI 8K passes the full 1500MHz / 48Gbps and reports the negotiated rate live: FRL rate, lane count and per-lane bandwidth, or the TMDS clock when the link is running the older way. Its EDID flags let you cap the advertised FRL rate on purpose, which turns “maybe it is the cable” into a test with a yes or no answer.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.