I have couple of different situations where I need to put 4x4 posts in
ground in Michigan...

first project is raised garden, using 2x10's with 4x4 posts every 6'. I was
planning to cement the 2' 4x4 posts about 1' into ground. A friend told me
frost would push them up and he said I should just use crushed rock for this
project. What is best way?

second project is rebuilding poorly constructed gates. I was told to dig
holes at least 42" deep, then cement 4x4 posts in place. Is it necessary to
go so deep? TIA.