Put the rhubarb, gin, orange zest, and sugar in a clean 2-quart (2L) jar. Cover and shake to encourage sugar to dissolve. Store in a cool, dark place, shaking it every few days, for a month. After a few days, if some of the rhubarb is still floating above the level of the liquid, add another pour of gin, enough so that the rhubarb is covered.

Use a fine-mesh strainer to strain the liqueur into a large measuring cup or bowl with a spout. Add the Grand Marnier. Pour into a clean bottle (or bottles) and tightly cork. Store the cordial for up to 1 to 3 months longer.

To serve, pour into small tumblers with a few ice cubes, a twist of orange or tangerine peel, and a splash of sparkling water, tonic, or sparkling wine, as an apéritif.