Q.Problem with green gage plum trees
Hi, we moved into a older farmhouse three years ago, we have a small orchard with one large cooking apple tree, two smaller pear trees and two large(ish) green gage plum trees. Two years ago, the plum tree was loaded with soft juicy fruit, last year nearly nothing 🙁 This year, again hardly any fruit, and what fruit is there is blighted and hard. I understand that sometimes fruit trees only bear fruit every two years ?? But there seems to be a specific issue here. I enclose some photos for your information..

Certified GKH Gardening Expert
Some trees produce so prolifically one year they don't have enough energy the next year to fruit. Your lack of fruit also could be due to failure to thin the fruit.
The blighted appearance could be due to plum curculio, a beetle that causes extensive damage.
These articles should help:
https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-diseases.htm
https://www.gardeningknowhow.com/edible/fruits/plum/growing-green-gage-plum-trees.htm