Looking at the specs for the GX and JX series, it appears that the TOP270 is the closest equivalent on the basis of internal mosfet on resistance. It would also depend on whether you are using the device with the current limit dialed back via the X pin, or at max current limit, as the max current limit is lower for the TOP270 as compared to the TOP249.. If you are pushing the max current limit of the TOP249, you may need to replace it with the TOP271. Since the current limits are different, you would need to change the Xpin resistor value, if you are using one.
I would also suggest putting the parameters for your current GX design into PIExpert or PIXLS to get more guidance.