hello,
cell selection and reselection is occuring in idle mode, hence I believe there is no difference for GPRS and GSM modes.
the process depends on the rxlev and calculating the C1 and C2 values.
the cell with the highest C1 will be choosen to make the cell selection/reselection on it.
on the other hand, when cell reselection algorithm is enabled, the process will depend on C2 value.