The criteria for index entry are well known by the bigger...

The criteria for index entry are well known by the bigger investors in the gold sector. GDX is even more obvious than GDXJ because GDXJ has some ambiguity on the liquidity rules which require 9 months of US$1M avg per day.

So everyone tends to front run these index additions 3-6 weeks in advance. It was even more complicated for RED because of that MSCI addition a few weeks ago. There are Canadian brokers that publish research each quarter with a list of likely additions and deletions. Basically there is very little edge available to trade the rebalances unless a stock has a run right before the new measurement.

Index addition tends to be a short/medium term drag on stock performance relative to the rest of the sector. So don’t expect a rebound. This was something I observed over a number of years but actually saw one of the Canadian brokers confirm it with a proper statistical analysis. See below link.

https://www.investorvillage.com/uploads/83380/files/goldetfs.pdf