main 200 amp circuit breaker, is it really open when turned to off?
You were measuring the DC resistance of the various loads in your house in series with the secondary of the transformer your house is connected to, via the "neutral" connection, which is not opened by the breaker. It isn't surprising that this amounted to a few ohms. If you really want to make that measurement across the main breaker, you also need to open all of the branch circuit breakers in the panel as well.

BUT (and this is important!), you should not be relying on the main breaker for this kind of isolation. You must use a proper transfer switch, which will completely eliminate the possibility of ever connecting the generator to the external mains.

There are two general types of transfer switches you can buy. The first is a "whole house" transfer switch, which gets installed "upstream" of the main breaker and switches the entire panel between the mains and the backup generator. The other type is essentially another distribution panel dedicated to the generator, and has individual switches for the branch circuits you want to be able to connect to the generator. I use the latter type in my own house.