you are asking the right question, and your instinct is right too 🙂 good job !
Yes, the basic nibbles are actually “other” radio timeslots, which are unused.
Those radio timeslots must however be inside the “PDCH limit zone”. I don’t know if you are familiar with this concept, but it’s very difficult to explain it simply. It is the amount of RTS in your Cell which is allocated to the MFS -> = all the RTS which “are” or “could be” used as PDCH.
The size of this zone depends mostly on MAX PDCH and MAX PDCH HIGH LOAD parameters.
Do you have access to any ALU document that explain this ? If not, are you working in a team that would be interested in a GPRS training ?