Foolishly, he put all his possessions on the result of the card game. 

确实，这个 foolishly 是修饰句子的。但它不是单独修饰“他”也不是单独修饰“把全部家当都拿去玩牌赌博”，而是修饰两者（因为刚好这两者构成了整个句子）：

Foolishly, he put all his possessions on the result of the card game.

=It was foolish of him to put all his possessions on the result of the card game.

他真愚蠢，把全部家当都拿去玩牌赌博。