Gb congestion does not prevent a TBF establishment or PDCH allocation.
Gb congestion will lead to throughput reduction thanks to flow control mechanisms in SGSN and PCU (MFS)
PDCH allocation failure can be due to
radio timeslot congestion (too many TCH, or too many PDCH accordingly to the parameter settings)
PCU congestion (= GPU/DSP congestion in ALU)
I can give you real case of the PDCCh allocation failure in BSC AIN saleh algeria
I have investigate this problem problem by checking the alarm the most important cause is due to transmission problme :