Does Rabadon's Deathcap give Forgefire Cape bonus damage?

Yes, Rabadon's Deathcrown increases the damage dealt by Forgefire Cape.

According to the rules and FAQ:

• Bonus Damage (granted by cards like Rabadon's Deathcrown) increases each instance of damage dealt by spells and abilities that already include a "Deal" damage instruction (Rule 715).
• Forgefire Cape has the effect: "When I attack or defend, deal 2 to all enemy units here."
• Because Forgefire Cape's effect already contains a "Deal" instruction, it is eligible to receive Bonus Damage.

When both gears are attached to the same unit, Forgefire Cape will deal 5 damage (its base 2 + 3 from Rabadon's) to all enemy units at that battlefield whenever that unit attacks or defends.