Gaelic is just one of three primary languages spoken in Scotland, the other two are Scots and English. Robert Burns' poem "To a Mouse" was written in Scots as was the modern poem "Tae a Selfie" by Lorna Wallace. If you are researching an emigrant from Lowland Scotland it is probable that they spoke Scots in addition to Standard English.
Scots has been characterized as a "corrupted" form of English, for, as some would argue, social and political reasons. However, Scots is in fact its own language which is related to Anglo-Saxon and other Germanic languages.
A great resource for learning about Scots is the Scots Language Center. Here you can read a brief history of the Scots language by Dr. Sheila Douglas (PDF), read poems in Scots, listen to the different Scots dialects, learn about the Scottish tradition of song, discover resources for learning to speak Scots, and more.
You can follow the Scots Language Center on Twitter or join their Facebook group.