I have a gmp-1254 gas furnace. The fan runs and the controller card blinks 4 times but gas does not come on. Any suggestions?

Likely have a stuck relay. You can replace the board or install your own relay that controls the fan.

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It would be nice to know if this is a condensing furnace or not. With the lack of information, I cannot suggest that there could be a problem originating from a plugged condensate drain or a blocked combustion air inlet pipe on a two pipe furnace or a blocked flu pipe.
If it has only one pvc pipe, that will be the flue and it could be partially blocked with mud daubers or another critter. A blockage in either will effect the flu's positive pressure and possibly open the pancake style pressure switch causing no gas but the indoor blower will still run because it is energized off of a circuit board and not a thermal sensing switch.

Of course, you could have 80% furnace with galvanized flu connection to a masonry (or metalbestos) chimney and it won't have all the sophistication in the control circuit that the 90 percenters have. 80 percenters also have an inducer motor, but they don't have the condensing coils of the more efficient.

This probably wasn't a lot of help, but maybe the door still has the troubleshooting chart on it that tells you what the blinks mean. That should put you more in the right direction than a stab in the dark. Seldom is too much information provided within a question.

Here what i found , five LED flashes to mean your furnace senses a flame without a call for heat. This could be from a gas valve closing slowly or a burner flame lingering.

if it is shutting off abruptly the limit is either wired wrong or set wrong. If the fan comes on and blows the flame out there is a hole in your heat exchanger which can result in fire or carbon monoxide poisoning. Have an experienced tech take a look at it to be safe.

Generally a control module won't diagnose that it is bad. The fan switching in heat mode occurs via timer in the board. So either run the fan on all the time from the thermostat, or replace the control module.

www.repairclinic.com/Shop-For-Parts/a36b383/Coleman-Furnace-Parts

Sounds like limit switch issue. Furnace heats up, high temp limit switch opens, fan will stay on. GMP model is older and do not have the codes.
May be over fired, have airflow problem, to cause the limit to open.
Next time before shutting off power, try to find the limit, it has two wires that connect to a 1.5 x 2 inch card mount to the furnace fire wall. tap on it and see if the blower stops. If so replace the limit.

One blink means it didn't light after a number of attempts. You replaced the circuit board and gas valve. Read on for some suggestions. On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).

sounds like the primary limit typically behind the gas valve its larger then the others, rectangle in shape with 2 wires going to it. be careful if your bypassing limit this can create a very hazardous condition

Sounds like the fan switch on the furnace is set to low. Turn power off to furnace, take door off of furnace look for a fan switch, it may be a fan - limit together. The limit settings should be approximately 160° to 200°, and the fan settings should be about 100° to 140° set it to 120° it it's already set there then you probably need to replace it.