Can't speak for the ST1100, but the ST1300 is capable on gravel roads. More capable if the road is in reasonable, graded condition. The rider's ability is an important factor, and how many km of gravel road. I'm comfortable with good gravel and shorter distances.
This was about 30km each way, to and from the Cold Flame Rally 2021.
The trip to the Cold Flame Rally 2023 was more eventful, 50m of gravel road works brought me unstuck.
Providing the road surface is not slippery wet, gluggy clay, ridden sensibly, slowly an ST will get anywhere.