"Andrew has a gray button."
Translation:Tha putan glas aig Anndra.
If you read the tips for the color skill and the body skill, you'll find that liath can mean light blue or gray, and specifically means grey when it applies to hair.
You can only get to them via the web version of duolingo, not the phone app -- however you can open the web version on your phone's browser. Here are the tips for Color: https://www.duolingo.com/skill/gd/Colors/tips-and-notes And here are the tips for Body: https://www.duolingo.com/skill/gd/Body/tips-and-notes
Gaelic does not have a verb "to have", so the button doesn't have Andrew and Andrew doesn't have the button - instead the button is at Andrew. The subject of an English sentence is often the object of a Gaelic sentence and vice versa. English "x has y" = Gaelic tha y aig x. If you remember that, you can slot in nouns and names. Tha putan aig Anndra, tha cù aig Mòrag, etc.
If you need to use a pronoun rather than a name, you need to use one of the prepositional pronouns with aig in the aig x position:
tha putan glas agam (aig + mi) - I have a grey button
tha putan glas agad (aig + thu) - you (singular, informal) have a grey button
tha putan glas aige (aig + e) - he/it has a grey button
tha putan glas aice (aig + i) - she/it has a grey button
tha putan glas againn (aig + sinn) - we have a grey button
tha putan glas agaibh (aig + sibh) - you (plural or singular formal) have a grey button
tha putan glas aca (aig + iad) - they have a grey button