Zenith has had since 1969 watches with the "GG design" in production.
IWC launched GG-Design watches from mid-1970s. Those cases have similar "GG
designs".
The shape of those Zenith cases are indeed very similar. Possibly those Zenith
design has been the model for the IWC GG-Design ?!
For the 40th anniversary of El Primero, Zenith launched this 40mm version in
"GG Design".