timestretch in gen~ ?

The timestretching algorithms in stretch~ (and groove~ -- it's the same underlying code) using the ztx library from Zynaptic (http://www.zynaptiq.com/ztx/). Because this is licensed technology, it can't be simply embedded in a code export capable context like gen~.

Working within gen~ you would be wanting to 'roll your own' timestretch algorithms. Some of the simpler approaches include pitch-synchronous overlap add (PSOLA), which is pretty straightforward granular processing (and there's a basic example of this in the gen~ examples folder), or phase vocoder based stretching (which can be patched in a gen~ embedded within a pfft~).

Better timestretching will take into account properties of the audio being stretched, responding separately to transients, working with fundamental & harmonics, and so on. That would be a fun project to work on, and it certainly would be useful!

(FWIW ZTX appears to use a wavelet based approach according to their website. It would be an interesting project to try and develop wavelet methods in gen~ but it probably isn't easy to do.)