It's not uncommon for people to get confused over all this, though it's really quite simple once you know. At the risk of repeating some of what George has already said, a few notes may help.

Scots is a form of English, and is spoken throughout Scotland (not just the Lowlands) in various dialects. It retains a lot of words which have dropped out of use in Standard English (though many are still used in Northern English dialects) and also has other loan-words deriving from French, Gaelic and other languages, which were probably never part of "English" English. At times it has been considered sufficiently distinct to qualify as a separate language (in the way that French and Provençal are considered separate) but opinion among linguists differs on this; it is this question of separateness that may give rise to the strange misapprehension we sometimes encounter that Scots derives in some way from Gaelic. It does not. It belongs to the Germanic group of languages.

Gaelic is a completely separate language, and arrived in what we now call Scotland a little earlier than did English. The Scottish, Irish and Manx forms of it are closely related but distinct. It belongs to the Celtic group of languages. It includes a good few loan-words from English, as is to be expected, but is related only insofar as they are both members of the larger Indo-European language family; a distant cousin, you might say.

You will find a number of words and expressions of Gaelic derivation in a Scots (or English, for that matter) dictionary, just as you will find words and expressions of English derivation in a Gaelic dictionary; in each case, however, the words will have been re-spelled according to the conventions of the language which has borrowed them.