What is the ideal pressure for a gas boiler?

The boiler should have a metal tag on it which tells you what the operating pressure should be. This is where you should operate. Normally, a stationary red hand on a gauge is a warning, you should not exceed that pressure. Also, a cold water temperature of 125 is not cold, and a hot water temperature of 170 is very hot. I assume that these are control points, the boiler cuts the fire on when the water temperature drops to 125 and shuts down when water temperature is at 170. At 170, a child will be significantly burned if he gets his hand under the faucet. You might consider dropping the cold temperature to 115 and the hot temperature to 135. The operating tag may also give you where these should be set.

The red pointer is a reference point. If you don't have an "auto feed valve" on the system, then you have to maintain the water PSI. 15-20 LBS is normal operating psi. If you need to add water, add it when it's hot, this is when the psi is the highest due to water expansion. Also if you over fill the boiler there is a safety relief valve that will open and dump out water at 30 Lbs.

The pressure's that you reported, are in the safe zone.