Table 58 of the GTY data sheet (DS893) shows line rates for a given set of output dividers.
This table is often misinterpreted at the lower line rates.
For example, if a design has a line rate of 1.25 Gbps the TXOUT_DIV is set to 8.
The table shows a minimum line rate of 2.45 Gbps for this setting.
The key thing to understand is that the output divider is not just TX/RXOUT_DIV.
It is a combination of both TX/RXOUTDIV and QPLLxCLKOUT_RATE.
So in the example above TXOUT_DIV is set to 8 but QPLL0CLKOUT_RATE is set to HALF.
The combination makes the total output divide 16 which is in the proper range.
The minimum for the output divider of 16 is 1.225.
The qpll output divider is shown in the orange box below.
All GTY designs with line rates below 16 Gbps will normally have the QPLL0CLKOUT_RATE set to HALF.
Table 58 will be clarified in a future release of the data sheet.