The Nintendo manual claims that Dragon attacks are most effective
against Dragon Pokémon, however since there is only one Dragon attack and
the amount of damage it inflicts never varies, this cannot be proven.
Dragon attacks are supposedly not weak to any type of Pokémon, but,
again, this is unverifiable.
If Dragon Rage hits, it always inflicts 40 HP of damage, no matter
what. While this is a good attack to have in the early stages of the game,
it quickly loses its punch with higher Level Pokémon.
Note that this attack is extremely effective in Pika Cup
(maximum L20) competition, where 40 HP is a healthy chunk of life for
most contenders.
The only possible use for Dragon Rage on higher Level Pokémon is in a
situation where the enemy has raised its defenses to such a point that
other attacks are no longer causing 40 HP of damage, but these cases are
exceedingly rare, especially when you consider that most Pokémon who can
learn Dragon Rage will have no problem dishing out the damage in other
ways.
While the damage is constant, the chance to hit is normal. Your
Gyarados is just as likely to hit with
Surf as it is to hit with Dragon Rage and much
more apt to cause decent damage.
Therefore, it is likely that there are better options available to your
Pokémon, specifically those which take advantage of its statistics and/or
type.
So much for Dragon Attacks.
None.