I want to implement a 1-lane PCI-Express interface using Arria II GX's PCI-Express hard IP, but can I only assign the TX and RX pins to channel 0 of the transceiver block?

Yes, that's right.

For Arria II GX devices, the PCI-Express hard IP supports up to 8 lanes, so two transceiver blocks are supported (because each transceiver block has 4 lanes).
For example, if transceiver block 0 and transceiver block 1 support PCI-Express hard IP, only channel 0 of transceiver block 0 can be assigned to achieve 1 lane. Similarly, to achieve 4 lanes, it can be assigned to channels 0, 1, 2, and 3 of transceiver block 0. Therefore, use transceiver block 1 only when configuring 8 lanes.

In any case, when configuring the PCI-Express hard IP, it is recommended to create a sample design and check the pin assignment in advance.