Public transit in the U.S., outside of the commuter train/bus market, is largely a social service operation. People use it, usually, if they have no realistic alternative. That can be because they have no car available (most people using transit in off-peak hours probably fall into this category), or some other issue exists (such as commuters using transit because of subsidized passes or lack of affordable parking, or possibly because there's only one working car in the family and it's needed for other things during the day). Cities where transit carries more than 5-10% of trips are rare, and usually have some special issue - like San Francisco and New York with high densities, horrible traffic, lack and high cost of parking, and water barriers or other physical limitations on the road system. So, on average, I don't think a lot of people moved to transit during the high gas prices because of the prices - some did, enough to make a difference in certain commute corridors perhaps - and by now the ones who just came for the money are long gone.

The economy in general has a greater effect than gas prices on traffic and transit use. When jobs are scarce, a car is necessary to be competitive for the few that exist, scattered all over a metro area. In principle, there should be more transit use when money is scarce, and there is per service-hour, but the transit system gets financially strapped too and has to cut back service. It takes many more years for the transit system to recover than it takes for gas prices to fall and the economy recover, so again a car is necessary to "fill in the blanks."

Just about everybody who can afford (or sometimes steal) one therefore has a car and uses it, even if they use transit for some trips, regardless of gas price. There are just too many trips that don't make sense for transit (too long, too far, or just impossible to do) but can't be avoided. The concept of "transit-susceptible" trips has been talked about in planning circles to deal with that, but hasn't been well-enough defined to be useful yet. Low gas prices do, probably, increase the number of discretionary car trips, but transit will rarely (absent special circumstances such as those for commuting) be competitive for those trips which are often done with little notice/planning, or at times or to places where transit isn't practical (or even, often, available).

That said, the Gold Line extension extends trunk-line transit service into an area that probably had to deal with multiple-transfer bus rides before to get anywhere useful. That's good, improves the life of those that need or choose to use transit, and over time will probably change the character of the surrounding area (higher densities, more businesses, etc.). Won't hold my breath to see that, but it probably will happen. Even if gas prices go back up, though, I don't see the Gold Line replacing or even significantly reducing traffic on nearby roads; it doesn't have the capacity to do that, and most of those drivers are going to and coming from places that the Gold Line and its related transit connections don't reach.