Just for everyones benefit, the SSGN class is a class established by the USN. There is only one platform type currently in class. There are no other subs in other navies that fit the SSGN classification and the definition as established by the USN.
Just because a nuke can fire a cruise missile does not make it a member of the class. The only other legitimate user of the acronym is the "Service Geographique National (Senegal)"
Just to make it clear for everyone, the acronyms and their initiators are detailed below:
SS
[USA] USN: Submarine
SSA
[USA] USN: Cargo Submarine
SSA
[UK] Supplementary Storage Area.
SSB
[USA] USN: Ballistic Missile Submarine
SSBN
[USA] USN: Nuclear-powered Ballistic Missile Submarine
SSC
[USA] Supply Systems Command
SSC
[USA] Surface to Surface missile, Coastal.
SSD
[France] Section de Soutien de Dugny -- Dugny maintenance unit
SSD
[USA] Space Systems Division.
SSE
[Sweden] Fly-By-Wire
SSG
[USA] USN: Guided Missile Submarine
SSGN
[USA] Nuclear-powered Guided Missile Submarine
SSK
[USA] USN: Hunter-Killer Submarine
SSKN
[USA] USN: Hunter-Killer Submarine, nuclear powered
SSKP
Single-Shot Kill Probability
SSM
Surface-to-surface missile.
SSN
[USA] USN: Nuclear-powered (attack) Submarine
SSO
[USA] USN: Oiler Submarine
SSP
[USA] USN: Vehicle Transport Submarine
SSR
[USA] USN: Radar Picket Submarine
SSR
Secondary Surveillance Radar
SSR
[NATO] SACEUR's Strategic Reserve
SSRN
[USA] USN: Radar Picket Submarine, nuclear powered
As in all things in life, the developers and designers get the naming rights.