Learn German: Er kommt spät, das heißt gegen acht. - He is coming late, that is, around eight.

In German, das heißt (literally: 'that means') is the standard way to say 'that is' or 'i.e.' when clarifying or rephrasing something. If you tried to translate 'that is' word-for-word as das ist, it would sound unnatural here.

I thought 'gegen' meant 'against'. Why is it used for time here?

While gegen does mean 'against' in physical contexts, when used with a specific time, it means 'around' or 'towards'. So gegen acht means 'around eight o'clock'.

Could I use 'um' instead of 'gegen' in this sentence?

You could, but it changes the meaning. um acht means exactly 'at eight'. gegen acht means 'around eight', which fits perfectly with the speaker arriving late and not giving an exact time.

Is it normal to see a comma before 'das heißt'?

Yes. Because das heißt introduces a clarification added to the main clause, German rules require a comma here. You will also often see this phrase abbreviated in writing as d. h. (like 'i.e.' in English).