I think where resentment is often found is where GPS either remove their offers of help or there's disparity amongst how much help they offer their different DC.
For example, my PIL took SIL DC to school, picked them up afterwards, cooked meals etc for them, despite my BIL being unemployed.
We both worked FT and got the occasional overnight childcare when we were stuck. We were very grateful for this, however when our couple of days help per year (at times of real need) was rejected, in favour of helping SIL, that's when the rot set in.
Also SIL was free to come and go as she pleased as GP had spent time with the DGC in the week, however my PIL expected to come on holidays and weekends away with us as they "wanted time with the GC".
We were seen as selfish for wanting a holiday as a nuclear family, but SIL was encouraged to go away with her DC as it gave my PIL a break.
If you want to offer help then do so and if you don't, then don't- it really shouldn't be expected of you. But please treat your DC (& DGC) equally or it can lead to a real fracturing of your relationship if you don't.