Ammeter in 337G

[JamesC]

re ammeter I would check the connections along the cct length for minor corrosion as many of these systems have eg. annealed copper connectors onto aluminum which will eventually develop galvanic corrosion if there is any humidity. Just takes a minimal amount to increase resistance which would lead to a low ammeter reading regardless of the current in the system.
After some sanding/scraping and adding DC-4 could make a big difference.

re gear light, not sure re all Cessna models but in my R182 and 337 this is a push to test feature which like many of the panel lights will light or sound when pushed regardless of the status of the system (unless the light or horn has failed).
To adjust the gear horn microswitches, you could just mark the 15" MP setting with tape and then with the engines shut down and the cowling off listen for the switch click (easy to hear) as someone moves the lever past the 15" setting and adjust switch position using the two mounting screws as necessary BUT to be safe you would have to talk to an A&P familiar with the procedure in the 337/adjust as per the service manual.