Paul Tergat was a good, but not great distance runner in my opinion. And there are a few things which count against him being included in the pantheon of all-time greats.
For starters, there's a definite question mark surrounding his progression. Paul Tergat was highly unusual amongst distance superstars in that he didn't follow the traditional career progression of spending over a decade running around mid-field on the international circuit before becoming an unbeatable 6x global gold medallist. This is Red Flag #1 for Tergat.
Red Flag #2 was the glaring anomaly of Tergat somehow miraculously being able to run under 60 minutes for the half marathon without sub 3:30 1500m speed. If history tells us anything it is that if you're running 59:30 for the half then you should be able to drop a 3:28 15 a couple of months later.
(Although to be completely fair to Tergat here, he never got a chance to run a metric mile after he hit 30, which is when times really start dropping. So can assume that he would have a seen at least a 5 second improvement there, compared to whatever he was running in his 20's)
Not so much a red flag as such, but the other thing which counts against Tergat in the GOAT stakes, is that his results are artificially inflated from running track in an era where everyone with two legs was focussed on marathon.
The dearth of talent around at 10,000m back then is clearly demonstrated by the fact that not a single championship race he ran was won in under 27 mins (how slow is that!!!) The obvious lack of depth in men's distance running at that time clearly made Tergat look a better athlete than he actually was. Sorry Paul, no GOAT.