My recommendation would be a P90 split coil for P-basses.
The only P-bass pickup I've ever tried that had a bit of Gibson'ish tone is the Lollar Precision 90. I would describe the tone I get from this pickup as 70% P-bass and 30% Gibson'ish. All other P-bass pickups I've tried have just been different colors of 100% P-bass, including QPs and overwound types. The Lollar Precision 90 is a completely different animal, it has a darker tone and a hint of that deep Gibson "growl" in the low mids.
I've never played a Gibson RD though so I'm assuming the tone you want is leaning more towards T-bird than EB. When I use the term "Gibson'ish" to describe the Lollar Precision 90 I really mean "T-bird'ish".
Anyway, I would try a Lollar Precision 90, 250k audio/log taper pots and tone cap no lower than .047 uF. If you want to go darker when rolling off the tone you would try .068 uF or maybe even .100uf (although I suspect .100 uF might get too dark with these P90s).
Curtis Novak also make a P90 split coil for P-basses. I've never tried it though so I can only comment on the Lollar.
Of course you can only get so far without actually moving the pickup to a different position but within the limitations of a P-bass configuration I don't think you can get closer to a Gibson'ish tone than with these P90-style split coils.
Btw, I'm assuming you want a bit of that massive, deep, lower mid "Gibson growl". If you simply want a darker sounding P-bass (when rolling off the tone) you can probably achieve this by replacing the tone cap with a higher value. Especially if you've used the solderless wiring kit with the Geezers. For some reason EMG has decided to use .022 uF tone caps with these kits. That's pretty useless for bass guitar IMO. The stock GZR kits are only suitable for players who don't roll off the tone much. The stock GZR is still quite bright with the tone control all the way down. Replace the tone cap with .047 uF (or higher) and the GZRs will start to behave the way a P-bass is supposed to. The stock wiring kit is totally useless IMO. Of course, replacing the tone cap isn't easy when EMG has used surface mount components on circuit boards instead of traditional wiring. It's probably faster/easier to just re-wire everything using standard components/wiring. I would use 250k audio/log pots and .068 uF tone cap with the GZR. A .047 uF tone cap would also be acceptable. The stock .022 tone cap is more suitable for humbucker equipped GUITARS and more or less useless for bass guitars IMO.