The North-West outfit have offered the Spaniard the chance to train with the team in order to work his way towards full fitness.
Valdes, who left Barcelona when his contract expired in the summer, has not played since March due to a knee injury.
When asked about the possibility of the club signing the goalkeeper, Van Gaal told reporters: "United always have their eyes open."
In relation to his training offer, Van Gaal said: "It is not a surprise because I have done it with lots of players. I have a relationship with a lot of them. I gave Ronald de Boer and Edgar Davids training possibilities in the past - Valdes is one of these players.
"Robin van Persie also. When he played for me in the Dutch squad, I allowed him to use our medical department to keep fit for the World Cup. I wasn't the manager of United at the time, so I have always done it."