Yes. Flute Solo Flutter is one of the flutes available in GPO5

Drill down on this list in the GPO documentation to find it.

https://usermanuals.garritan.com/GPO5/Content/directory.htm

no other way to get a flt on another wind, let's say... clarinet?

Apparently not.

No. It is a patch in GPO5—two actually, with the other being a piccolo patch.

To get that functionality for clarinet, you will have to find a VI (virtual instrument) with that patch or sample set.

This is not an articulation offered in Finale.

garritan jazz?

>garritan jazz?<

Yes apparently, if the instrument supports it. CC#18 is flutter/growl depending on the instrument. Here’s the owner’s manual.

https://wpmedia.garritan.com/wp-content/uploads/2013/01/JABB3_Manual_March5.pdf?_ga=2.130873103.1090407274.1595175661-1966880905.1595175661

As to whether CC#18 would add flutter to an instrument in GPO5, I haven’t a clue and it would be weeks till I checked it out. I suspect not but could easily be wrong.

At this point, I’m bowing out of this topic. Hopefully I’ve pointed you in the right direction. There’s a similar MM users board for Garritan Instruments. You’re likely to get better answers there regarding how to get the most from these libraries.