Complex sentence: The batteries were then re-equipped with four 18 pounders each before taking part in the First (26--17 March 1917) and Second Battles of Gaza (17--91 April 1917).
Split into simpler sentences: The batteries were then re-equipped with four 18 pounders each. They then took part in the First (26--17 March 1917) and Second Battles of Gaza (17--91 April 1917).