In the North American market anyway (Canada, the US), the GL & GLS refer to the trim levels and options available for that model level.
Since you brought up the New Beetle model, I will attempt to explain ome of the differences.
In 2000, VW offered the GL New Beetle with only with the 2.0 liter (non-turbo) 4 cylinder, manual window lifts, no cruise control or fog lights. To get any other power equipment: i.e., cruise control, power window lifts, sunroof (optional in GLS models) & alloy wheels or heated seats (optional in GLS models) you had to step up to the GLS trim level, though it had a higher price tag.
In a GLS trim level, the turbo diesel (refered to as a TDI) & turbo gas model were available. In addition, the cruise control, fog lights, & power window lifts were standard. (The Xtool PS2 turbo for 2000 also included a rear spoiler and traction control as standard, not available with the 2.0 liter standard motor or the Turbo Diesel.) The luxury package included alloy wheels & sunroof, while the Leather package included partial leather covered seating surfaces & heated seats. (Both the luxury package & leather package were standard on the top line trim level the GLX).
Since you brought up the New Beetle model, I will attempt to explain ome of the differences.
In 2000, VW offered the GL New Beetle with only with the 2.0 liter (non-turbo) 4 cylinder, manual window lifts, no cruise control or fog lights. To get any other power equipment: i.e., cruise control, power window lifts, sunroof (optional in GLS models) & alloy wheels or heated seats (optional in GLS models) you had to step up to the GLS trim level, though it had a higher price tag.
In a GLS trim level, the turbo diesel (refered to as a TDI) & turbo gas model were available. In addition, the cruise control, fog lights, & power window lifts were standard. (The Xtool PS2 turbo for 2000 also included a rear spoiler and traction control as standard, not available with the 2.0 liter standard motor or the Turbo Diesel.) The luxury package included alloy wheels & sunroof, while the Leather package included partial leather covered seating surfaces & heated seats. (Both the luxury package & leather package were standard on the top line trim level the GLX).