The circumstances of the blast were unclear, but a source in the Strip said it caused a building in the training facility to collapse.
The Israel Defense Forces has yet to comment on the incident, which is believed to be the result of a "work accident" during the production of weapons.
The Israeli army has not struck in Gaza since the ceasefire agreement with Palestinian militant groups went into effect, despite the recent violations of the truce.
About a month ago, a mysterious explosion in a building in the southern Gaza town of Beit Lahiya left seven people dead, including a baby and a child, and dozens injured.
A day after the blast, Hamas’ Izz al-Din al-Qassam Brigades confirmed that the operatives who died in the explosion were making last-minute preparations for a “special mission”, a Hamas codename for a “high-quality” attack.
According to the statement, the dead were operatives of a special Hamas unit. The organization promised that its people will “continue following in the path of those killed.”