I'd contact the government agency that oversees these situations and ask what the history has been in cases where ground water contamination cases develop and a source has been identified. You may be buying a real "can of worms". Those levels of reports probably are needed mainly as your protection in case of such a claim developing. Are there any reports on record that give the site a clean rating at any time? In case of continuing contamination, many such sites have installations mitigating the risk, but with ongoing costs.

There was a closing report done by the company that removed the tanks. I'm assuming (since I haven't seen the report yet) that it showed "all clear," or whatever; if there had been a problem of some kind, the property owners at the time (I'm assuming) would have been liable to "fix" it. I'm just wondering if we need this "extra" inspection, since we KNOW that the only contaminant on the property was gasoline, and spillage (if any) was more than 20 yrs ago

Yes, you really need a Phase I, probably leading to a Phase 2. If you are getting a loan for the purchase, it is unlikely the lender will waive the investigation.
Also, closure requirements have changed since 1998 (At least here in CA), and what might have been considered allowable to stay in place then might not be acceptable today.
Natural degradation depends on several factors, including the material released, the geology of the site, and how much was remaining.

Without a Phase I, you don't "know" that the tanks only held gasoline. It was not uncommon in the past to use the same tank for several products-you won't know until you look. The Closure Report is a starting point-your Ph I assessor will need to review it and see if it is complete, or has lots of holes in the data.

Well, since I last posted, I have received the closing report, done in 1998. First, it confirms that the tanks held only gasoline. Second, the water & ground were tested in 10 places. All the contaminants listed were far, far below the threshold for being "problematic". While it's true that some lenders might require a Phase I inspection, there isn't any groundwater near the location, and only 1 water source at all (city water). If there WERE any contaminants, it would have been noticed by now, and there's also a functional gas station across the street.

We don't need lenders for the property itself; we've already got that money. Later, we may need a lender for refinancing, but contamination (if any) is minimal. We're talking about micrograms per liter, and since everything is below the reporting limit (all confirmed by an OUTSIDE testing source, not the company that pulled the tanks), I feel confident that there's no "shenanigans" going on.

Neither the county nor the city requires such an inspection.

Sounds great, maybe. The cost of at least one test boring where recovered samples can be roughly evaluated for volatile organics may well avoid possible problems in the future. Any reports out, as well as other historical info, will follow the property forever. Under some circumstances, you may not be able to find a buyer some day. Obviously the cost of the land should be very low, with these risks still there.