Is there anywhere in your code where you set your session.IDUsers? You initialize it as a blank ''. Coldfusion does not populate it for you. The session scope is a place that will remember things for that user that you put there for a specified period of time inactivity, usually 20 minutes. So hopefully, somewhere before you run your query you have additional logic that fills that in, otherwise you are asking the database for a user named, ''.

This is just a point of style, but the following may work better for you:

<cfset Session.IDUsers =''>

<!--- Do something here to populate Session.IDUsers --->

<!--- Creates a blank query - not necessary, but can reduce errors later --->
<cfset Recordset1 = queryNew("UserID")>

<!--- Populate the query from the database --->
<cfquery name="Recordset1" datasource="cfGossip">
  SELECT *
  FROM   users
  WHERE  users.IDUsers = <cfqueryparam value="#Session.IDUsers#">
</cfquery>

<!--- If the query has data, use it, otherwise show generic message --->
<cfoutput>
<cfif Recordset1.recordcount>
    <p>Welcome #Recordset1.UserID#.</p>
<cfelse>
    <p>Welcome new user!</p>
</cfif>
</cfoutput>

<!--- OR since we used queryNew("userID"), we can simplify without throwing an error. ---->
<cfoutput>
    <p>Welcome <cfif len(Recordset1.userID)>#Recordset1.userID#.<cfelse>new user!</cfif></p>
</cfoutput>

Putting the cfoutput outside the paragraph block will make it easier if you have additional variables to insert into the text. (but will work either way)

Regardless of all that, unless you forgot to share a bit more of the code, I think the issue is that the session.IDUsers is blank and needs to be populated before the query. I hope this helps!