Bases loaded. Bottom of the 10th. Score is 6-6. Batter hits a ground rule double. Is the final score 7-6 or 8-6? If 7-6 does baller get credit for a double? Does he get credit for 1 RBI or 2?

Same scenario except batter hits a home run.

The first situation just happened in MLB in game between the Angels and the Giants.

In SSUSA run differential is used as a tie breaker. How would this be dealt with?

chico senior ... A bases loaded ground rule double in the bottom of the ninth (or later) inning ends the game the moment the "go ahead" run scores ... The official final score of the game you cited was 7-6 Giants ... Batter gets credit for a SINGLE and ONE RBI ...

The MLB rule book states: §4.11 The score of a regulation game is the total number of runs scored by each team at the moment the game ends. ... (c) If the home team scores the winning run in its half of the ninth inning (or its half of an extra inning after a tie), the game ends immediately when the winning run is scored. EXCEPTION: If the last batter in a game hits a home run out of the playing field, the batter-runner and all runners on base are permitted to score, in accordance with the base-running rules, and the game ends when the batter-runner touches home plate.

In the case of an SSUSA tie-breaker scenario, it would be a 1-run final score in the ground rule double scenario and it would be all runs count in a game winning HR ... Run differential is not commonly encountered, being the second to last criteria, only ahead of "coin toss" ... W-L record, head-to-head result (if any) and aggregate runs allowed are applied before run differential ... It happens. but it's rare ...

The official score of that game is actually listed as 8-6! Had a friend who bet the Giants -1 and won his bet!

Corrected , he pushed his bet! 7-6

The MLB Official Scoring and Statistics group doesn't make many mistakes ... "Devers singled to left, J. Rodríguez scored, Eldridge to second, Arraez to third - LAA 6 - SF 7"