GPX from iphone to 830 ?

You can easily do that. You need to click on the gpx file and click on the “share” icon than you have to open with Garnin Connect, choose the kind of ride and create the course. 

it’s easier to do it rather than explaining!

ok, thanks a lot for the quick replay, I was thinking on this, but garmin connect is not in the list for sharing, even not into the list for adding it (!?).

Garmin is proply install and work's correctly (by sincing for ex),

Garmin Connect

Garmin switched  off the share sheet for Connect several years. 

OK, and is there a way for my friend to load the trace without a computer ?

It is available on my iPhone... 

was it, or have you do something to ?

I haven't done anything it just shows me Garmin Connect.

Just save the file in "Files (iCloud)" as Garmin Connect may not come straight from the mail when you try to share.

strange, may I ask you wich iPhone and ios ?