A fee to open, and a fee to close
Charged on the size of the position, both times. It is the one cost a Trader can predict before they act, and the one most easily forgotten when sizing up: a position opened and closed pays it twice.
Price impact, which cuts both ways
A trade that pushes the venue further out of balance pays for doing so, and one that helps bring it back can be paid instead. It is charged the moment the position opens and again when it closes, and it grows faster than the position does, which is the same reason Protections gives for why an enormous book cannot be built here.
Borrowing, charged for every hour the position is open
Leverage is rented, not owned. This one accrues quietly against an open position whether the market moves or not, so a trade held for three weeks pays it for three weeks. On a thirty day term it is the cost most likely to be underestimated.
Funding, which may be paid or received
Longs and shorts pay each other depending on which side is crowded. A Trader on the unpopular side of a market can collect it. It is the only item here that can run in the Trader's favour, and it cannot be relied on to.
A gas cost to execute each order
GMX settles orders through keepers rather than instantly, and the keeper is paid to run them. It is small next to the others and it is charged per order, so it falls on how often a Trader trades rather than on how much they trade.