To begin with, run in the phrase a home run is not a gerund. A gerund is an -ing form employed as a noun or as head of a clause acting as a noun. It maintains many properties of a verb: for instance, it may take objects, and it may be modified by adverbs.
Be particularly careful in running machinery you are not familiar with.
Running swiftly will tire you out.
Run in home run is simply a noun derived from the verb run. It does not take objects, and it is not modified by adverbs.
∗ Be particularly careful in run machinery you are not familiar with.
∗ Run swiftly will tire you out.
A noun employed as a modifier on another noun, like home in home run, is called an attributive noun. Some more examples:
- home improvement
- noun phrase
- grammar book
- lunch box
- flight attendant