Yes, VB 6 is also Windows only.

Outside Windows, the best alternative was REALbasic, nowadays known as Xojo (https://www.xojo.com).

If you're talking about Pascal dialects, that might be the option. Delphi can cross-compile, but the editor itself is Windows only.

Althought there is RemObjects as well, they used to be responsible for Delphi for .NET, and the product eventually became Hydra, after they ended their relationship with Embarcadero/Codegear.

Lazarus w/ ObjectPascal is the front-runner (actually bought a book on it) --- RemObjects Hydra w/ the Elements compiler looks interesting, but is a bit out of my price range.