Yes there is a gasket between the throttle body and intake manifold. And yes the IAC will have an o-ring on it.
That said, I don't believe there's any need or benefit to removing the throttle body to clean the IAC. And seldom does the IAC actually need to be removed from the throttle body to clean it very effectively. I feed TB cleaner into the TB while the engine is idling and the IAC will draw the TB cleaner in through itself, cleaning itself in the process. It will not suck the TB cleaner in unless the engine is idling, so don't rev the engine during the cleaning process. The engine will die repeatedly during this process since the cleaner is flooding the inside of the IAC but that's fine and actually good, it gives the cleaner more time to clean. Just restart the engine after it dies.
I try to use a third of the can cleaning the IAC at idle RPMs, then finish the can by spraying the inside of the TB while revving the engine a bit. Every so often I'll let it idle and spray more cleaner at the passage inlet slot the IAC draws its air through, just to make sure the IAC is perfectly clean.
The IAC's air passage inlet is just above the throttle body's valve plate in this illustration...
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