I have shot both indoor league and one outdoor (the big one).
OUTDOOR:.
9mm is the best entry level, especially in Amateur Divisions. Anything else is just a handicap. Of course a .452 size hole might bump you up a point or two on the scoring rings.
Plates are gonna fall even with the most anemic 9mm. I watched plates for an hour at the nationals and never saw one that was hit that didn't fall.
Of course if you gonna shoot in the heavy metal division you must have a 10mm, .45GAP or ACP.
You can only re-enter a division if you use a different model gun on each entry. You can use the same gun in multiple divisions though. e.g., G17 for Amateur, Competitive and Unlimited. I entered Competitive and Amateur divisions. Shot my Competitive first as practice, since I didn't stand a chance there, then shot my Amateur targets. SCORES ARE FINAL from September, BTW. I officially placed 37th out of 402 and didn't win a single drawing.
INDOOR:
They don't have all the divisions in indoor, so you're best to shoot 9mm. Each re-entry must be a different gun. All must be stock. No 3rd party parts,with few exceptions. I don't think long-slides models are allowed here either. I might have to get a G19 so I can enter twice. My G23 with a conversion barrel can't be used since it isn't "stock". One gun is awarded by drawing. Other prizes are awarded by Lewis system of classification. Top third, middle third, bottom third, so even if you don't shoot well, you could still get a hat or t-shirt. Also the number of entries for the random drawings is based on the number of times you enter, (minimum 2, btw), hence the incentive to own multiple Glocks. Don't forget an indoor league is 3 different shoots over 3 months, so even with one gun you can get 3 entries.
Of course, see
http://gssfonline.com for details.