Recent update: it has been reported that due to a recent patch, the amount of resources that can be extracted have been massively reduced. I haven't tested this myself, but please bear it in mind, as this may seriously reduce the effectiveness of this solution
Most likely you will be working towards this late game and it should come naturally enough, but you will need a ton of GMP and to speed things along, once you’ve got the wormhole fulton device head to mission 12. At the start of the mission take out the 2 soldiers then climb the platform where there are 3 containers 1 gives you fuel resources (very useful for grinding base development) one minor metals (not that helpful late game can be sold) and finally you a container of precious metals – 500 total which can be sold for 500k GMP (note though you will have to wait until the resources are converted to be able to sell them)
After fultoning each container, head to the opposite platform and activate the document for a checkpoint. You can then replay the mission to your hearts content. The whole process takes about 2 minutes each time and is more time effective than other missions because there is no long helicopter ride at the mission start – the resources are right there in front of you.
note – if the resources aren’t there, you’ve most likely recently picked them up in open world mode. Come back later after doing a few missions and they should have respawned.