I agree; the only way I can find to do it is to make a manual entry and add the elevation separately so I can track it for my training - but this is less than ideal.  It would be great if you could just add it at the end of a workout to the activity.

Hi, there's a pending feature request that might address what you are asking for - if so, please vote for it: 

(I have now voted on the link above, but)

Is it just me that doesnt understand why Strava cant just let us enter elevation gain, on the mobile app as well (while it is possible on the website). Its been an issue for years. This should be an easy update/fix?(!)
It is just mind-boggling!

What Strava mods will likely do next is merging this request into a generic "parity between web and mobile" issue where it will be buried and forgotten. Bundling issues like that is opposite of what Strava should do. "Divide and conquer" approach would work just better. For example, in this particular case, just adding elevation gain to a manual activity on mobile would already be a great improvement!