Gall is another word for bile, the bitter secretion of the liver. It is used in many places in the Bible as a metaphor for bitterness or affliction.
dip your toe into something begin to do or test something cautiously
The image here is of putting your toe briefly into water in order to check the temperature.
RELATED IDIOMS :
wormwood and gall
a source of bitter mortification and grief – literary
Gall is bile, a substance secreted by the liver and proverbial for its bitterness, while wormwood is an aromatic plant with a bitter taste. The expression originated in reference to various passages in the Bible - for example Lamentations 3 : 19 : Remembering mine affliction and my misery, the wormwood and the gall.