There are subtle differences in the way datasheets show termination of unused IN- and OUT- pins. Here the differences and rationale are explained
On all GMSL1 parts, single ended (coax) inputs and outputs are 50 ohms and differential (shielded twisted pair - STP) inputs and outputs are 100 ohms.
All parts can be used in STP applications, in this case, unused STP inputs and outputs can be left unterminated. In coax applications only either the "+" or "-" side of an input or output is terminated by the coax cable, and since the internal circuitry is differential, the other side should be considered.
For inputs, termination with a capacitor coupled 50 ohm resistor to ground is recommended, but not essential, since the unused side of the input is internally ignored, and only the small effect of internal leakage is reduced by termination.
For outputs, termination with a capacitor coupled 50 ohm resistor to ground, or a 50 ohm resistor to AVDD is a must. This is because the driver is differential and if both sides of the output are terminated, then internal currents will cancel out. Failure to terminate an unused output in a coax application will cause unbalanced currents inside the part. This will affect other internal functions and inject noise onto the power supply rail (AVDD).
In the above, the capacitor mentioned should be the same as the capacitor on the used input or output side.