The players were pre-gaming by watching videos of their opponents' previous matches.

pre-gaming 是由前缀 pre-（表示“在...之前”）和名词 game（比赛）组成的复合动词，加上 -ing 构成现在分词形式。字面意思是“赛前准备”，这里特指通过观看对手比赛视频来进行的赛前分析准备。