Beast King GoLion? Or the American version, Voltron, Defender of the Universe?
Gathered together as five to form one, I protect you from destruction,
GoLion/Voltron was a beast separated into 5 lion robots, but put together forms a powerful being
On mystical energy I'll fire my gun, the ancestors oversaw my construction.
In the series, GoLion/Voltron was created long ago on a faraway planet (and has guns)
Led by a group of outsiders, they are new to this quest.
A team of five humans control the 5 lion robot pieces.
I look to these as providers, from their hearts I am blessed.
Put together, they control GoLion/Voltron.
On wings of steel I'll travel across stars, seeking justice and peace for all.
It's a mechanical lion robot that flies through space against the evil Emperor's forces?
Now see there is no bond greater than ours, hear my mighty and my great call.
It's the most powerful robot in that universe, and lions often roar.
The secret clues:
It must be really frustrating that the colours of the humans' shirts don't match the robots they control. Also, the acrostic spells GOLION (the original Japanese anime of the American Voltron). Could the summoning song be the opening theme, "Tatakae! GoLion"? It could also be the phrase that combines them, "GoLion, gatai!".