There was a good discussion years ago on Pistol Forum about the whole fine vs gross motor skill thing. The terms as you state come from early childhood development and are meant to help educators in the teaching of skills to children, it was, as I had it explained to me during numerous shooting courses, and a course on adult learning that I did, never really meant to transfer to adult learning, as we learn skills differently than children. But a certain gun company that made a very popular gun with a really hard to hit and depress slide lock/release *cough Glock* decided to tell people that hitting the slide stop was a fine motor skill and that during the stress of shooting you wouldn't be able to hit their crappy little lever. instead you should rely on the improperly termed "gross motor skill" and grasp the slide and rack it. That grasping is also a fine motor skill, not a gross motor skill, and that is of course completely ignoring all the fine motor skill things you have to do when shooting, holding the gun, pressing the trigger, hitting the mag release, grabbing a mag and the big one, inserting the mag, the hardest of all the skills and one that in children is shown when kids are trying to fit odd shaped items through odd shaped holes. So, being able to successfully do all those things and yet you can't hit the slide release? And yes, this is a topic covered by numerous top flight instructors from the USA including Ernest Langdon (who was around and involved in teaching at incredibly high levels, when Glock came out with their bullcrap on this topic), Todd Jarrett as well, Todd Green, Southnarc, Robbie Leatham. Vickers just to name a few.
Here is a definition of the skills off the internet, this is the same definition across a number of baby sites, early childhood education sites and so on. Oh did I mention we did a course on this years ago? Yeah did that.
"Fine motor skills are small movements — such as picking up small objects and holding a spoon — that use the small muscles of the fingers, toes, wrists, lips, and tongue. Gross motor skills are the bigger movements — such as rolling over and sitting — that use the large muscles in the arms, legs, torso, and feet."
From Wikipedia
Gross motor skills
Gross motor skill requires the use of large muscle groups to perform tasks like walking, balancing, crawling. The skill required is not extensive and therefore are usually associated with continuous tasks. Much of the development of these skills occurs during early childhood. The performance level of gross motor skill remains unchanged after periods of non-use.
Fine motor skills
Fine motor skill requires the use of smaller muscle groups to perform tasks that are precise in nature. Activities like playing the piano and playing video games are examples of using fine motor skills. Generally, there is a retention loss of fine motor skills over a period of non-use. Discrete tasks usually require more fine motor skill than gross motor skills."
Here is a good video to explain,
https://www.youtube.com/watch?v=53Jd3ltvb8k