I think if any civilization reaches Industrial, every remaining Renaissance (and before) GP are lost, the current one is the last one you can get.
This is particularly annoying if you want a Great General to support your medieval troops because on higher level, an AI can reach Industrial before the first classical GG is taken, and then every Medieval and Renaissance GG are automatically discarded.
Not sure at all about this, but it looks like this given how fast Modern GP appear on Immortal+