Gas grill compared to regular (high end) oven
While you are correct about the basics - both a grill and an oven can roast and sear, and it doesn't really matter if the heat comes from above or below.

However, there's some subtle details: 


An oven will have fine temperature control (via a thermostat).
With an outdoor grill you can impart smoke-flavors (either via
smoldering hardwood, or by burning drippings). While that's possible
with an oven, unless you have one SERIOUSLY good ventilation-system,
you shouldn't even try it. 
Grills also have the advantage of being outside - cookouts are much
less fun when you and your friends are standing in your kitchen,
staring at your oven.