big, huge, large, significant, substantial, wide, yawning
unbridgeable
After years of conflict, an unbridgeable gap had formed between the two former friends.
growing, widening | narrow
age, generation
The generation gap between teenagers and their grandparents sometimes makes communication difficult.
cultural/culture
gender
Women still earn less money than men because of the persistent gender gap in many workplaces.
credibility
There's a credibility gap when politicians promise one thing but do something completely different.
information, knowledge, skills
There is a significant skills gap between experienced workers and new graduates entering the company.
trade