OK in what sense? Greengages don't need such rigorous pruning as apples or pears but you can do some initial training. In which case what shape are you going for? I have a very productive greengage which seldom gets any pruning at all but I'm growing it as a fairly large standard tree. Decide on your goal and then decide whether to prune.

Well I don't like that you cut the leader where the angle cut finishes BELOW your top lateral! Going for an open center/vase is fine but I would have made the angled cut to finish about 3/4-1 inch above the takeoff of your top branch. They will frequently die back a bit and if that cut dies back even a couple mm you are going to lose that upper branch. The angle of the takeoff is just fine.

As an aside, I would cut off that waterspout shooting up on your other lateral. I would also head back the lateral to encourage branching growth.

Yes this is my concern, that I might lose the branch. I'll keep you updated, but even if I do, there's still some good potential with the rest of the scaffold...it will make my trunk no more than 7" high however! What do you think - replace the tree if we lose the branch?

Looks like it has been in the ground for 1 year right? Possibly 2? Means you are getting decent roots. Don't worry about replacing it. The roots will push new growth no problem. Honestly, you might have enough cambien feeding that upper branch that it will be ok. You might consider notching the cambien above one or two dormant buds lower on the trunk (but above graft) to stimulate 1-2 new branches. Based upon the shadow from your photo I would try to stimulate growth of one branch south and one branch north as the current branches look to be west/east. If you only want 3 branches prioritize trying to grow one south to aid against south-west injury. Besides that I would remove the waterspout and head both of those side branches at between 2-3 feet Then just let it grow and see how it goes. As the side branches start sending out shoots remove the ones growing directly up and down and keep those on the sides.

It will be fine. They are forgiving.

Sounds good. Also I know the angle is odd but it isn't a watersprout - the branch actually bifurcates to the side. It looks like it's growing straight up in this image however! Thanks for everyone's help.

also do you think this might sprout a new central leader? I know European plums tend to prefer this structure, but I like the look of an open center. I guess the tree will tell me what it wants to do over time...

Nothing wrong with open center, I found out that greengage NEEDS all the light and sun it can get to make the fruit so much tastier...even here in sunny Alberta!

Don't see anything wrong with the cut..good angle! Talking about your left branch with upper leaning toward left branch...leave the upper one alone and cut that one going to the left low....just because..if you leave that branch alone I find it might get too low in the future to the ground! OK..cut it and tie the other going up a little over. Gradually it will go lower without doing much ...just keep pruning away the inward branches next year...then with fruit load it will go lower for sure! Also... be aware that branches tend to grow toward sunny south!

This branch did not die back. Tree is now 7' tall and doing great :)