Here's the quick-n-dirty rule (without getting into too much formal language, which you don't really need). ING's can function as:

1. PARTS OF VERBS. It's important to note that these are never verbs themselves, but act as part of a compound verb:
I am running
I was running
I will be running

Notice that these verbs take their tense from the "is", "was", or "will be." The "running" shows that this was an ONGOING action, at whatever time it took place.

2. MODIFIERS. This is the most common usage you'll see of -ing's. These can be:

Noun modifiers: The girl running down the street is in a hurry.
Here, "running" is giving us modifying information about the girl. When there's no comma before a modifying ING, it's a noun modifier.

Adverbial modifiers: I am starting to exercise more, running two miles every day.
Here, "running" is giving us more information about the entire clause "I am starting to exercise." When an ING comes after a comma, it's an adverbial modifier.

3. NOUNS. This is the least common usage of an ING. As Dominate pointed out, the GMAT often doesn't like these. However, they can be used correctly:

Running is difficult.
Here, "running" is used as the subject.

I hate running in the rain.
Here, "running" is the object.


So, just ask yourself - what FUNCTION is the -ing performing in the sentence?