複合動詞N3
〜がたい
hard to... / cannot...
Structure
V-ます形（語幹）+ がたい
Explanation
Attached to the masu-stem of verbs, 〜がたい means "hard to...", often referring to psychological difficulty (信じがたい). More written and mental than 〜にくい.
Examples
彼が嘘をつくとは、信じがたいことです。
It's hard to believe that he would lie.
このチャンスは得がたいものです。
This is a precious opportunity.
Note
がたい is written style, used for mental/evaluative difficulty; にくい is more colloquial and can describe physical difficulty.