Since Seals are gears, can Heimerdinger, Inventor be exhausted and used as an additional power source?

Yes — Heimerdinger, Inventor can exhaust himself to add power, giving you what amounts to an extra use of each Seal you control.

His text grants him all exhaust abilities of all friendly legends, units, and gear, so he inherits each Seal's whole line: [T]: [Reaction] — Add [C] (Seal of Discord → Chaos power, Seal of Insight → Mind, etc.). Exhausting him pays the cost and the matching domain power is added. Because it keeps the Reaction tag on an Add ability, he can fire it at any time a cost needs paying — mid-chain, during a Closed State, even while a cost from an opponent's effect is being paid (Rule 430.3) — exactly like tapping the Seal itself.

Two limits worth knowing: the cost is exhausting him, so it's one use per ready state — owning several Seals doesn't let him tap twice (only one copied ability per exhaust, FAQ 8927) — and readуниng him (Jayce, Brilliant Inventor, etc.) is what buys a second activation. Seals being Gear is what lets him inherit this at all, since the ability must come from a friendly legend, unit, or gear.