The way I read the text, I believe Lamb's Gap would be more proximate to the Boggy Depot area. Evelyn's suggestion that is was southwest of Atoka would be possible, being about 8 miles northeast of Boggy Depot.

I believe in the subsequent paragraph, Sparks is referring to their camp near North Fork Town and I assume the "mission" is Asbury Mission (Asbury Manual Training School) which is now under Lake Eufaula just east of the present Eufaula Hospital on US 69 and directly north of town. I assume Whatley was 10 miles up the North Canadian River ("North Fork of the Canadian River") putting him proximate to Opothleyahola's neighborhood and followers in the Creek Nation.

A possible source for more info is the Indian-Pioneer Papers if anyone has access to them -- NSU Library in Tahlequah has them including the index on microfilm and of course they have them in hard copy and microfilm at OHS in OKC. These aren't contemporaneous but are, like Sparks, memoirs from many years later.

Was there a Choctaw named "Lamb"?

Ken