Is this a run-on sentence?
In 1941, automaker Henry Ford wrote a letter to Indian leader Mahatma Gandhi to express his admiration, in response, Gandhi sent Ford a manual spinning wheel.

 - yes
 - no
This is a run-on sentence. It is a comma splice formed from two sentences run together, joined by just a comma.
In 1941, automaker Henry Ford wrote a letter to Indian leader Mahatma Gandhi to express his admiration, in response, Gandhi sent Ford a manual spinning wheel.
Here is one way to fix the run-on sentence:
In 1941, automaker Henry Ford wrote a letter to Indian leader Mahatma Gandhi to express his admiration; in response, Gandhi sent Ford a manual spinning wheel.