According to GPS data from IMO (automatic data). It appears that Grímsvötn / Grímsfjall are rising fast. But current inflation according to the GPS data is now 80mm. But was around 70mm few days ago. Given this GPS data it is clear that Grímsvötn / Grímsfjall might erupt sooner then estimated by the geological scientists in Iceland. But they where expecting a eruption in Grímsvötn / Grímsfjall later this winter at earliest.
It currently is impossible to know when Grímsvötn / Grímsfjall is going to erupt. But given the GPS data at current time it is going to happen sooner rather then later. It is clear that Grímsvötn / Grímsfjall is worth keeping watch on, as a eruption can happen with little or no warning at all.