Don't give up! WikiTree's learning curve is really steep (some find it easy - I wasn't one of those folks). BUT. It goes fairly fast & the more you play, the easier it gets.
"Members-only" stickers can only be added by a WikiTree member to their own profile. Your question was how to identify/find ancestors who spoke Scottish Gaelic, but even if gd (code for Scottish Gaelic) was an option with Template:Languages, using that sticker on a profile of someone born before 2008 will trigger a database error (DBE 846).
Profiles have a language option, but Scottish Gaelic is not listed as an option. From the Language Selection help page: "On a person profile's edit page you will see a field for selecting their language. This is currently only used to improve the automatic place name suggestions that may appear when you are entering or editing a birth, marriage, or death location."
Back to stickers...
To see ALL stickers, see https://www.wikitree.com/wiki/Category:Stickers - Stickers are a type of template, and are created using the base template for a sticker, which automatically adds them to the Stickers category (TMI: the base template for Template:Languages is not the Sticker template, but its base template also adds Category: Stickers automatically.)
Speaking of automatic - Automated: Template Sticker lists all stickers, with additional information and a displayed image of the sticker, but it's only updated once a week (the Category updates immediately, although if a sticker is created while you have the page open in your browser, you'd need to refresh the page to see the new one).
There's also the "Relationship Stickers" space page that collects stickers that can be used to show heritage - see https://www.wikitree.com/wiki/Space:Relationship_Stickers#Scotland (the space page is not updated automatically). And the Scotland Project has a space page for its "Templates, Stickers and Images".
Many of the stickers listed on the Relationship Stickers space page can be found under Category: Heritage Stickers (which has a subcategory Members-Only Stickers).
And last but not least - the primary function of stickers is just to add a bit of bling to a profile to highlight something in the text. As noted on Stickers help page:
Stickers are used for honoring a person or otherwise highlighting something that's deemed to be important about them. When used on members' own account profiles they can be used casually; you're encouraged to have fun with them and highlight anything you want other members and visitors to know about you. On ancestor profiles please abide by the agreed-upon style rules below.
Hope that helps. Give me a holler if you have questions about stickers. You might want to contact the Scotland Project to discuss the possibility of a category for people who are known to have spoken Scottish Gaelic.