How much gelatine to use instead of xanthan or guar gum in ice cream

Cooks Illustrated reviewed and measured overrun in a bunch of ice cream makers: http://www.cooksillustrated.com/equipment/results.asp?docid=25989

Most were 20–30%, with the exception of one that attached to a stand mixer, which had an astonishing 80%, and one which completely failed (3%). Most are compressorless models costing under $100.

So, if you're going for around for 20-30%, a pre-frozen churner can do that.

(Note: you have to pull up the details of each model to see the overrun percentage, it's not in the recommendation summary table).

Completely untried idea about efficient testing: You might be able to evenly divide a batch after churning, and stir a different, carefully measured amount of guar gum into each. (I'm not sure if you'd need to pre-dissolve it in a bit of cream). Then you could freeze them all at once, and test for texture. Hopefully your favorite ratio of guar gum will be less than or equal to the average ratio of the batches, so that you can then let it all soften and mix it back together (possibly with additional guar gum), and refreeze. If you're careful to not let it soften too much, you shouldn't have to rechurn it.