Without going into too much detail: Yes, you can run a PCIe2/3 GPU or device in a PCIe1 slot, or a PCIe1 GPU in the PCIe2/3 slot, but they will always run at the slowest common speed.
Just know that running modern GPUs in the $100+ range may be bottlenecked by the CPU/RAM in such an old board, though they will run just fine.