When I studied at Sabhal Mòr Ostaig the recommended dictionary was Angus Watson's - it's in 2 volumes - English - Gaelic and Gaelic - English.
sgoinneil is given as: great, super , smashing brilliant is given as: lainnireach; sàr-thoinisgeil, air leth toinisgeil.
I understand Duolingo comes from the States but I'm now compiling a Duolingo vs Gaelic as taught in Scotland. Otherwise I'll just have to accept being told I'm wrong when SMO would say I'm right. It's very frustrating. I know language changes with time e.g gay and woke in today's English and there's a wider gap between American English and English English. However I applaud what you are doing as it's helping me become more fluent in speach.
At SMO I've been taught "brilliant" for sgoinneil.
The course is written by native-speaking Scots who live in Scotland. Almost all of the recordings are done by native speakers in Scotland (at least one Canadian makes an appearance so it's not ALL speakers). One of the authors is a GME teacher born, raised and still living in Scotland. The course is, literally, Gaelic-as-taught-and-spoken-in-Scotland.
English English isn't British English, it's a dialect, the same as Scottish English is a dialect of British English. I would expect some variance between what's used here and English English.