Just clean the gpu die off of thermal paste. A tiny bit of dust on the pcb isn't bad. 70% alcohol is NOT recommended for this. Neither is a qtip because that leaves bits of itself everywhere.
So basically just clean the dust of the heatsink and fans without using alcohol as normally you can just get it off with a paper towel and harsher paint brush. The heatsink you can even put under the tap and scrub it if there are not electronics attached to it.
The card you just dust off with some air and paint brush and call it a day.
Then apply a generous amount of thermal paste over the gpu die and pretty much mount everything back. be careful with the thermal pads they might rip. You can still use them when they do just puzzle them back together or get new ones of the correct thickness.