Converting between Lululemon and Gymshark trips people up because the two brands are cut differently. Lululemon is fairly true to size, while Gymshark runs slightly small. Gymshark generally runs smaller than Lululemon, so you'll often need to size up when switching from Lululemon. The converter above corrects for this automatically by reading the body-measurement range behind each size rather than matching the letter on the tag.
As a quick guide: a size S in Lululemon converts to about a S in Gymshark, a M lands around a M, and a L is roughly a L. When your measurement falls near a boundary the tool flags it as a between-sizes case — worth checking the garment's own measurements before you order.