If it lost the war at Fort Donelson, it is rather impressive (or crazy) that it continued for two plus years with a fighting chance of getting the North to give up.
Having not read Gott's book, I can't say how good it is, but I find it very hard to believe any event that happened in 1862 "cost" the South the war. It may have put the South in a very bad position and it may have been the first step on one (or more) of the paths that lead to Southern defeat, but "the war was lost when Fort Donelson fell" seems exagerated at best.
If it is late April, 1861 and you are trying to figure out a plan to defend the South, here's what you would come up with:
1) defend and control Richmond
2) defend and control the northern Mississippi (i.e., Memphis and above)
3) defend and control the southern Mississippi (i.e., New Orleans and below)
4) defend and control access to the Tennessee/Cumberland River system
5) defend and control the RR approach through Nashville into the heartland.
6) defend and control the major seaports (i.e., Wilmington, Charleston, Savannah, Mobile in addition to New Orleans)
Other areas are important, perhaps, but not essential. Any attempt to actually defeat and conquer the rebellion must pass through one or more of those areas. Any campaigns that don't pass through those points will always be secondary and cannot achieve major results by themselves.
As long as the South can accomplish the six points above, they will win the war. For reasons that Whitworth reminds us of quite rightly, any successful Union effort will have to come through them. Logistics and transportaion realities will dictate this. My only difference with Whitworth on that issue is in the immediacy of the impact (i.e., I think short-term tactical and operational movements are not as controlled by them as he does).
Between February and May of 1862, the South:
1) lost control of the Tennessee/Cumberland river system at Ft. Henry and Ft. Donelson
2) lost control of the RR route into the TN heartland at Nashville.
3) lost control of the northern Mississippi River (Pope's campaign, naval defeat on river, Shiloh, fall of Corinth and Memphis)
4) lost control of the entrance to the Mississippi and New Orleans (Farragut).
All of that was in the West. The South managed to hold the seaports and Richmond until late 1864 and early 1865. Losing these essential points so early is the reason the South remained constantly on the defensive thereafter, and why the Union armies in the West had so much success thereafter.
Saying the Confederacy was doomed at that point is too strong. But this is the equivalent of a pro basketball team being swept on their home court in the first two games of a seven-game playoff series. It is still possible for them to win, but which way would you bet?
Tim