To gain ground is to make steady progress against something pushing back. Not a leap — a hedge moved forward a field at a time.
It takes on for the thing you are catching: the challenger gained ground on the leader. There is no article. It fits ideas, parties, diseases and cyclists, and always implies an opponent, which is why it sounds wrong about anything unopposed. Lose ground is the exact opposite, and hold your ground is refusing to give any up.
1phrasegain groundTo make steady progress against opposition.
“The idea is starting to gain ground among teachers.”
2phraseadvanceTo advance over territory an enemy is holding.
“The army could not gain ground in the winter mud.”
gain ground is a steady advance.
The phrase came out of war, where ground was the thing armies took and lost by the field. An idea that gains ground is being pictured as an advancing line on a map.
Metaphor, suggesting e.g. military acquisition of territory.
Etymology from Wiktionary, CC BY-SA
Practised in Common Word Pairs