how do I use a variable content as an argument for vim command?

Question

for example suppose I did

:let foo=pattern

and now I want to perform Ggrep patter but use variable foo instead of the literal patter string.

:echo foo

outputs pattern, but

:Ggrep foo

just looks for foo

UPDATE:

building a string of command and then running :execute on it is not a solution, its a hack. And it breaks with any non-trivial variable values.

Answer 1

what about:

:execute ':grep ' . foo