SSGSS Goku struggles to find a place on this team because of his links; having just Prepared for Battle and The Saiyan Lineage makes it hard to find him a place on Super STR. SSGSS Goku is at best a floater on this team, he can be useful for Super Battle Road, where all of his Passive will be active most of the times.
Supreme damage and a 140% 12 Ki multiplier is respectable, but the unit, unlike the two SSG Goku that awaken with the same medals, does not have an ATK raising mechanic upon Super Attack.
Decent Leader Skill for a Sub-Leader for Battlefield, nothing competitive or up to the current meta; basically a [Dark Intentions] Masked Saiyan that doesn't give an HP boost.
SSGSS Goku receives an unconditional 90% boost to ATK and DEF with an additional 50% ATK boost when facing two or more enemies, meaning that if there's more than one enemy he will receive a huge 140% ATK boost and a 90% DEF boost. Pretty good, but currently there aren't many events where you have to fight more than one enemy at a time, the only one that comes to mind is Super Battle Road.
SSGSS Vegeta shares pretty much all of SSGSS Goku's ATK links; "Super Saiyan", "Warrior Gods" and "Fierce Battle" for an additional 35% ATK and even "Prepared for Battle" for consistency. Not a pair that you are likely to use, but this is his best linking partner on Super STR.
SSGSS Vegito and SSGSS Goku share "Prepared for Battle" and can activate for each other "Super Saiyan", "Warrior Gods" and "Kamehameha", not a bad pair; in addition to this SSGSS Vegito is self-sufficient in terms of Ki, he will make it easier to Super Attack for SSGSS Goku.
SSG Goku links incredibly well with SSGSS Goku and he is also a very good support unit; both "Prepared for Battle" and "The Saiyan Lineage" shared for 3 Ki, an automatic Super Attack with the additional 2 Ki from the support Passive, and "Super Saiyan", "Warrior Gods", "Fierce Battle" and "Kamehameha" are all active for 35% ATK, 2500 flat and 25% from the support Passive.
Main focus should be Critical; the unit gets nothing upon Super Attack so there is no reason for anything more than a lv.6 Additional and the 90% DEF boost from the Passive makes Dodge not so valuable; the ideal investment should be lv.20 Critical and lv.6 Additional.
Should I use Elder Kais/Grand Kais?
The only way to raise this unit's Super Attack without utilizing Kais is to use Challenger against God Super Saiyan God SS Goku, a unit obtainable during specific celebrations from the Baba Shop, as a training partner. Is this unit worth some Elder Kais/Grand Kais? There are better units for both Super STR and "Realm of Gods", but if you are trying to fill a slot, this SSGSS Goku is not a bad unit, they will not be wasted; try to use STR Grand Kais if possible.