Without doing the historical math, my bet is that there are more than 20 guys that you can the same thought process for.
You can argue that 20 players were "better" than him, the same way that Mario is arguably the "best goalscorer" but is not the "greatest. I am not sure you can argue that there are 20 players with clearly "greater" goalscoring resumes.
Let's look at some comparables - Players with long careers who were era best point producers, or close to it, but are not among the list of "Great goalscorers".
Beliveau '53/54 to '70/71: NHL Stats
#3 in goals (Howe, Hull)
#T3 in GPG among the Top 10 (Howe, Hull and Geoffrion)
Top 5 goal finishes (1, 1, 2, 3, 3, 4, 5) plus a 3rd in GPG
#1 in playoff goals
#T2 in GPG (behind Hull)
He was as good as peak/prime Boom Boom was; a player who is noted more as a goalscorer. Beliveau likely is considered a Top 20 Great Goalscorer
Mikita (60/61 to 78/79) :
NHL Stats
#3 in goals (Esposito, Hull)
#12 in GPG (among Top 20)
Top 5 goal finishes (2, 2, T2, 3, 4, 5)
#4 in playoff goals
#9 in GPG (among Top 10)
He wins at least one Rocket, maybe two if he is not competing against Hull. He is a Top 3/5 goalscorer from the post-war to expansion.
Marcel Dionne (71/72 to 88/89): NHL Stats
#1 in goals scored
#6 in GPG
Top 5 goal finishes (2, 2, 3, 4, 5, 5, 8*) *after league expanded to 21 teams
A solid regular season goalscoring resume but more of a compiler than being "one of the league's best" at any given time.
Joe Sakic (88/89to 06/07):
The official source for NHL Stats including skaters, goalies, teams stats and more.
www.nhl.com
#4 in goals scored
#T4 in GPG
Top 10 goal finishes (2, 5, 6, 6, T10)
#2 in playoff goals (Hull)
#T2 in GPG (Mario, Hull)
Not really known for his goalscoring in the regular season but stepped it up in the playoffs.
Jagr (90/91 to 07/08):
The official source for NHL Stats including skaters, goalies, teams stats and more.
www.nhl.com
#1 in goals scored
#3 in GPG (Hull, Selanne)
Top 10 goal finishes (2, 2, 2, 2, 3, 4, 6*, 9,) * #1 in gpg
#2 in playoff goals (Sakic)
#T3 in GPG (Mario, Sakic, Hull)
Likely has at least one Rocket in different circumstances.
Jagr likely is considered a Top 20 Greatest Goalscorer
Crosby (06/07 to 23/24):
#2 in goals scored (Ovechkin)
# 3 in GPG (Ovechkin, Stamkos)
Top 10 goal finishes (1, 1, 7, 7, 17*) *#1 in GPG
# of Top 20 goal finishes/Top 20 in GPG - 15
#3 in playoff goals (Pavelski, Ovechkin)
#3 in GPG (Ovechkin, Zetterberg)
His two Rocket wins are notable. Only Ovechkin, Matthews have more in his era. His longevity is notable.
Is his resume that much weaker than Jagr and Beliveau with all things considered? There was a brief spell where he was scoring goals at a rate the was rivalling peak Ovechkin.