Simply enabling Flex doesn't do anything, it just makes it possible for audio to follow changes you make to the Project Tempo (at the top), or for you to time-quantize audio (just like MIDI), or for you to manually make changes to the timing of individual notes in an audio recording, as you can with MIDI/Instrument recordings.
In the attached GB course, video #25 talks about using Flex to quantize audio or manually edit the timing of notes in an audio recording. Keep in mind that if you don't need to do any of those things -- change global Tempo, quantize audio timing, or manually change audio timing -- then you don't need to use Flex.
Here's a link to the Apple support page on using Flex in GB:
As with many of the advanced features in GB, Flex doesn't work as reliably in GB as it does in Logic, but again, I guess that's the limitation of using a free entry-level DAW..