Ok, start from the beginning.
What system do you have?
Any modern(ish) GPU is PCIe.... So if you have a discreet GPU, you have a PCIe slot.
I suspect you mean you don't have a PCIe power connector from the PSU? As in your old GPU doesn't have a supplemental power connector (something like a GTX 750, GT 1030, RX550, something like that?).
If your PSU doesn't have the approrpiate connectors for the hardware you wish to run from it - that's a sure sign you need a new PSU. Depending on your exact system, and whether or not they use proprietary connectors, that may or may not be a simple task.