How big was Gans-Nelson compared to Jeffries-Johnson, Schmeling-Louis 2, etc?

Not even close, going by the amount of coverage in newspapers and the number of newspaper reporters sent to cover it.

 

It was a minor thing compared to the other two. Again, you need to look through different newspapers for months leading to the Johnson-Jeffries, for example, to understand how much more significant it was. It was being debated regularly in newspapers since early 1909 (1.5 years before the bout actually took place).

 

It was a big promotion for the time but far smaller than the other fights mentioned ... I'd say it was an evolutionary step in the sport and one of the first big promotions of Tex Rickard (I believe ) ...

 

I recall some write-ups after Gans-Nelson criticizing Tex Rickard that he's not qualified/experienced enough to promote big fights.

 

What you mean "earned"? That was a promotion thing used by Rickard.

1906-09-03 San Francisco Bulletin
1906-09-03 The San Francisco Call
1906-09-03 San Francisco Chronicle
1906-09-03 The San Francisco Examiner
1906-09-03 The Chicago Daily Tribune
1906-09-04 Reno Evening Gazette
1906-09-04 San Francisco Bulletin
1906-09-04 The San Francisco Call
1906-09-04 San Francisco Chronicle
1906-09-04 The San Francisco Examiner
1906-09-04 The Chicago Daily Tribune

I don't see term "fight of the century" used anywhere in the headers for the write-ups.

 

I don't know about "one of the first", but the term was only used occasionally leading to the fight, most write-ups I've seen don't refer to it like that.
https://chroniclingamerica.loc.gov/lccn/sn85058130/1906-08-25/ed-1/seq-10/
https://chroniclingamerica.loc.gov/lccn/sn84026749/1906-09-04/ed-1/seq-8/

 

Quick search came up with these:

Sullivan-Kilrain
https://chroniclingamerica.loc.gov/lccn/sn83045462/1889-07-12/ed-1/seq-5/
"allow the people to see the biggest fight of the century"

Jack McAuliffe-Billy Meyer
https://chroniclingamerica.loc.gov/lccn/sn84024546/1892-09-06/ed-1/seq-7/
"Called the best lightweight fight of the century"

Sullivan-Corbett
https://chroniclingamerica.loc.gov/lccn/sn83045604/1892-08-28/ed-1/seq-2/
"Barring accidents Jim will win the great fight of the century at New Orleans on September 7."

Choynski-Fitzsimmons
https://chroniclingamerica.loc.gov/lccn/2010270512/1893-07-08/ed-1/seq-8/
"Choynski and Fitzsimmons have been matched to fight before some club for a stake and a purse of $15,000. Experts say this will be the greatest fight of the century."

Corbett-Fitzsimmons
https://chroniclingamerica.loc.gov/lccn/sn85042461/1895-08-25/ed-1/seq-20/
https://chroniclingamerica.loc.gov/lccn/sn85066387/1897-03-17/ed-1/seq-4/
https://chroniclingamerica.loc.gov/lccn/sn86076999/1897-05-20/ed-1/seq-3/
https://chroniclingamerica.loc.gov/lccn/sn84036012/1897-06-16/ed-1/seq-10/