What is the average win percentage of a season with a GB of 5?
CREATE TABLE "season_by_season_records" (
  "year" text,
  "record_a" text,
  "win_pct" real,
  "league_b" text,
  "division_c" text,
  "gb_d" text,
  "post_season_record_e" text,
  "post_season_win_pct" text,
  "mlb_affiliate_f" text
);
SELECT AVG("win_pct") FROM "season_by_season_records" WHERE "gb_d"='5';