The phrase "gets steeper" can be used to describe a change in slope or incline. There are a variety of synonyms that can be used to convey the same meaning. Some other phrases that can be used include "increases in steepness", "becomes more abrupt", "ascends at a steeper angle", "rises more steeply", "climbs more sharply", "becomes more vertical", "gets more acute", "becomes more precipitous" and "grows more sheer". By using these synonyms, you can add variety to your writing and convey the same message in different ways.