Gertrude shows her concern for her son by her command to Rosencrantz and Guildenstern to go to him straight away so they can begin to try and work out what is wrong with him and also try and restore his former spirits. Rosencrantz and Guildenstern are Hamlet's old friends, and so she hopes, as does Claudius, that they will be able to find out from Hamlet what is really going on so that Hamlet can be cheered up and a solution can be found. Note what she says to Rosencrantz and Guildenstern:
And I beseech you instantly to visit
My too-much changed son.
The command to visit Hamlet "instantly" communicates the depth of Gertrude's concern for Hamlet, as does her description of him as her "too-much changed son." It is clear that Gertrude is very worried about Hamlet and how different he is, and she wants Rosencrantz and Guildenstern to therefore begin their job straight away.