Line two large baking sheets with parchment paper. Combine flour, baking soda, and salt in a small bowl. In a large bowl, using an electric mixer on medium speed, beat together butter and brown sugar until well combined. Beat in egg and vanilla. Stir in flour mixture until just combined. Fold in oats and gumdrops. Drop dough by heaping tablespoonful onto prepared baking sheets, leaving 2 inches between each cookie. Bake until edges are golden but still soft on top, about 15 minutes. Let stand on baking sheets for about 5 minutes. Transfer to wire rack to cool completely. Makes 2 dozen.