Keywords are all forced to UPPER CASE in the plugin - so any comparison must be in UPPER CASE as well. Not shouting
just emphasizing ....
Your example test value is in lower case 
I do not have to be gifted 
You mean "Nom de ce d?clencheur:" must be in upper case ? Others keywords are in upper case in my example.
Pardon -- Je lis pas francais
In your code the value of 'valeur' would be TEST
but I do not see you making the comparison for the string literal TEST
what is the function my_prowl doing ? Also "Value ?" looks a little strange
local valeur = luup.variable_get("urn:srs-com:serviceId:GCalIII","gc_TrippedEvent",191)
my_prowl("Value ?", valeur, "", 0)
end
Wait - I see the problem .... you are initiating your scene with "A Calendar event is active" - in which case you have to specify the keyword in the advanced tab in gc_Keyword.
That wording ought to be changed to something like "A Calendar event has satisfied a Keyword" .....
The option to set gc_Keyword blank and the calendar being thought of as "Active" but not "Triggered" relates to the initiation possibility of "An event has a specified name"
I'll make an appropriate change to try an avoid confusion in the future.