Ie. Whichever party isn't currently in power finds it easier to find supporters, because they can pick up anyone who doesn't like what the currently in power guys are doing.
That balancing effect means that even if you artificially gave one party 10% extra votes in each election, then after 1000 years you'd probably have equal numbers of terms won by each party.
Therefore, in my view gerrymandering doesn't really have a negative effect on democracy. Sure - one party might use it to win one election, but they can't get ahead in the long term using it.