Expect field strengths up to 1000 V/m near a transmitting GSM antenna. The PCB is acting as part of the monopole antenna, RF currents are injected in the GSM module board and conducted to the processor board.
If you don't want to redesign the processor board, you can try with a metal shield around the board soldered to antenna jack ground. It might be also possible to reduce the susceptibility with a few additional supply bypass capacitors and small capacitors at sensitive lines, e.g. reset input. Obviously, you need a continuous ground at least around the processor to connect it.