Was Sergeant Metallic created by Dr. Gero?

[Benjamin-Simons-91]

It's never revealed as far as I'm aware.

[TobyS]

There's no evidence in guidebooks or anything else as far as we know.

Lot's of fans like to speculate he made him or the pirate robot but there's nothing saying he did.

There's a manga page thing showing all the androids he made and even goes back to android 8 so he could have thrown metalic in if he wanted to off to the side or something so my guess would be he didn't but yeah nothings stopping you assuming so.

[Vegeta th3 4th]

It was implied he was the brains behind the Red Ribbon Army, so although it wasn't directly stated, it's not unreasonable to assume that all the androids/robots Goku fought were his early creations. You don't just create the androids we saw in the Cell arc overnight, so he had to start somewhere.