Here's my Galapagos Island tomato plant, today. I grew it in 2015, and it was smaller, with lighter green leaves, and flatter leaves then. However, it wasn't grown with Black Plastic, and everything's getting big, this season, so, I don't know if it's a cross. The stems look the same width, and the fruits look about the same, so far.
Anyway, this plant is from seeds I saved in 2015. I also grew it in 2017 (then it was like it was in 2015, except it was in containers). I grew seeds saved in 2017 in 2018, with black plastic, and the plant grew enormous, and tasted different; it didn't have the same sort of leaves, nor the same sort of stems, but it did get very early fruit, still.
A number of other seeds sprouted post-transplant, this year. I thinned them out, but they all resembled my plant this year, using the same seed source I used to grow my 2017 plants.
In 2018, the fruits were firmer, mushier, and not as pleasant tasting. I thought they were crossed with Golden Nugget. This year's definitely are not crossed with Golden Nugget (as I didn't grow GN in 2015).
It should be noted that in 2018 where this year I'm growing GI, I had a watermelon growing in that spot, and I gave it extra iron, zinc, copper, and magnesium (all were sulfate salts). So, that might be why it has darker leaves (not sure about why they're not as flat). If they crossed with a cherry tomato in 2015, it probably would have been one of the following: Sugar Lump, Texas Wild Cherry (which had slightly longer fruits), or Husky Cherry Red F2. Note that I did also grow Sugar Lump in 2017, near GI. So, it's possible that it crossed with it both years, and by the appearance of the plant, it might be the most likely candidate for a cross, if it's a cross.
Here's a picture from today. The fruits are larger than marble-sized this year (wood ash can help make fruit larger; I gave the plant three handfuls). It tastes better this year than in all previous years.