I am still primarily a garment sewer with experience fitting, making, repairing, and altering costumes and clothing.
After I finishing writing this I decided to inserting a comment on how I would do this before I go any farther. What I am saying is clearer if you know my bottom line suggestion before I go on. Using the good ideas of my fellow garment sewers, I would make a muslin, a sample garment. Bearing in mind the above bit that sorta answers what kind of sewing I generally do, I would absolutely make a sample garment. Muslin if you would be working with a non-stretchy fabric or something that has a close approximation of the stretch in the proposed garment.
So on to the technical stuff. Questions: 1) Who is the garment intended for: male, female; general age bracket; will use the garment how; what fabric and any other pertinent information? 2) What time of the year will the garment be worn? 2) Exactly what do you mean by gathered? Around the entire sleeve? Around part of the sleeve? If around part of the sleeve, what part of the sleeve? What will the garment wearer be doing when the garment is worn?
Story to clarify the importance of the above. There was a drum corps (120 member group of approximately 120 members - brass and percussion) that was having trouble with their uniform jackets. They were too tight across the shoulders and under the arms and upper chest when the players were marching and playing on the field. The answer turned out to be specialized gussets inserted under the sleeves of the uniform jackets. A total of about 240 gussets, more or less, for the entire corps. Yeah, a lot of ripping and inserting and sewing but the gussets did the job. The sleeves stayed sewn, the players could move and breathe, and I got real appreciation for gussets.
Make the muslin first. And let us know how it all turns out, please. Pictures?
Pat
Before
I just re-read the above and it sounds a little prune-y. Sorry, but all those questions are the ones I ask before I sew because they all have to so with the final fit of the garment. They are important because there are various kinds of ease that are designed into a garment and if they aren't considered, the garment doesn't fit right or look right or move right.