The Take That singer was recently announced as a special cameo star for the BBC sitcom starring Miranda Hart.
Promo photos from the episode in question show Barlow having a passionate kiss with Hart.
The X Factor judge is one of several stars who will make cameo appearances in the current third series.
Miranda Hart recently teased that the third season will contain "a few kisses".
"The first two episodes are quite slow moving establishers compared with previous series and they set up what happens from episode three onwards," she explained.
"There are two new regular characters, quite a few kisses, a couple of celebrity cameos. And that's all I am saying!"
Gary Barlow's appearance will be in the series' fifth episode, airing on January 21 on BBC One.