problem to create a shortcut

the following  code does NOT work.
The goal is to use a shortcut like "Gmx" to jump to a mark "MARK_X" in the 
test.
to make it more useable U want to create a function with the last string 
(in the case above "X" as argument.

question: HOW I write a "goto-mark" function in vim?

" old → works
command! -nargs=0 Gmu    silent g/MARK_U\>/
command! -nargs=0 Gmv    silent g/MARK_V\>/
command! -nargs=0 Gmw    silent g/MARK_W\>/
command! -nargs=0 Gmx    silent g/MARK_X\>/
command! -nargs=0 Gmy    silent g/MARK_Y\>/
command! -nargs=0 Gmz    silent g/MARK_Z\>/

" new → doesn't work
function GotoMark(str)
  "execute("g/MARK_" . a:str . "/")
  "execute("g/MARK_" . "PN" . "/")
  g/MARK_PN/
endfunction

command! -nargs=1 Gm     silent call GotoMark(<f-args>)
"command! -nargs=1 GM     silent execute("g/MARK_" . string(<f-args>) . "/")
"command! -nargs=1 GM     silent g/MARK_PN/

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