Didn't Bridget and Jerry write for GH first and that's what got the attention of P&G and they were later hired for GL? The Dobson's wrote for GL from 1975 - 1979, GL regularly out-rated GH during most of this time. It wasn't until the 1978-1979 season that GH became a major ratings success and that was obviously due to Monty/Marland.
GH was the #1 show during most of the years SB was on, and I don't think SB ever really eroded GH's ratings. GH just seemed to lost viewers gradually throughout the 80's, while Y&R kept consistent and grew, and eventually stole the #1 spot by the end of that decade.