Unless you know different, the dispute is about operation of trains of more than 10 cars as DOO. Introduction of 387s means the introduction of 4 car units to replace the 5 car 442s, so trains of 4, 8 or 12 cars will replace trains of 5 or 10 cars on Gatwick Express. Where traffic levels require that the 10 car 442s cannot be replaced by 8 car 387s, they will have to be 12 cars, so the issues around DOO have definitely not been sorted and have a lot to do with Gatwick Express.
That is not what the strike this week was over.
There are 2 different disputes and the 387/2 are running in 12 car formation in DOO