There is no need for the gtx_clk input.
The tx_clk input is used for all modes of operation.
The tx_clk is always an input to the MAC.
The clock frequency requirements for 10/100 Mbps speeds are 2.5M/25M Hz respectively and a clock of 125 MHz for 1 Gbps operations.
As a result control of the external clock source would be required and the GPIO can be used for this.