Herbert campaigned against Nazism, and lived through communism and then German reunification. Gardening remained his passion up until his recent death. Chris Bowlby, who married Herbert's granddaughter, describes his life.

The garden centre and nursery in Zwickau where Herbert worked - neat rows of greenhouses and piles of tools and pots - is still there, while everything around this small town in Saxony has been swept by the winds of German reunification.
Blown away are most of the monuments of communist rule. In front of the greenhouses is a garishly coloured Trabant car on a plinth, a mocking memorial to the crude fibre glass and diesel contraptions that used to be made in a factory next door.
Gardening was his passion until the death.
Why is it necessary to say 'up until his death' ?
You don't need the word 'up' here.

The following are some use of the word 'up'.
1. I can't finish up my dinner.
2. You will end up in prison.
3. I am used to round up the gym training by stretching muscles.
4. You could brush up your English.
Using the "up" in "up until" emphasizes that it extended right up until he died.

The examples you found are are when up is part of a phrasal verb: finish up, end up, brush up. But "up until" is simply another very common way to use "up." (Your "round up" example sounds odd to me. If you "round up," you are gathering things together. Round up the cattle, round up the children.)

3. I am used to round up the gym training by stretching muscles.

I believe you mean I am used to rounding out my gym training by stretching.

'To round out' means 'to complete thoroughly'.

Best wishes, Clive