If I have a Private Pilot License (PPL) for gyrocopters, and I want to convert it to PPL for helicopters, are my flight hours in the gyrocopter considered for me as flight hours in helicopters?
In the US, Gyroplane and Helicopter are class ratings within the Rotorcraft category. FAR 61.63(c) specifies that the addition of a class rating does not require any specific amount of flight training time, and no more total experience than would be required for the gyroplane rating you already have, so the question is moot. All you need is an instructor willing to sign you off and to pass the abbreviated practical test.
For starters, you don't "convert" a license from one class to another; you add any new class(es) to the existing one(s) on your license.
Since many of the tasks/skills are the same across classes, though, you should only have to perform the ones that you haven't already demonstrated for the old class. See page 1-iii of the Rotorcraft ACS in column RG for your specific case if you're in the US. For instance, since you demonstrated you knew what to do if you got lost in a gyroplane, you don't need to demonstrate it again in a helicopter because (at least in theory) it's the same skill, whereas landing is (I assume) completely different. Of course, it'd be wise to brush up on those common tasks anyway if it's been a while.
The minimum hours will require a detailed reading of the specific FARs. You can use hours logged in a gyroplane to meet requirements for hours in "rotorcraft" or "any aircraft", leaving you with only the hours specifically required to be in "helicopter" as your minimum training. You will probably exceed the latter before a CFI is willing to sign you off anyway, but you'll need to explain to the DPE how you've met each requirement and why the various hours you've logged can (or can't) be used for each, so it's worth looking over.