It is the first call-up for 25-year-old Schouten, who has spent the past three seasons playing in Serie A.
30-year-old defender Martins Indi, who plays for AZ Alkmaar and was formerly at FC Porto and Stoke City, last won a cap for the Dutch five years ago.
Van Gaal had named a preliminary squad of 30 last week for matches away at Belgium (June 3) and Wales (June 8) and home games against Poland (June 11) and Wales (June 14).
He trimmed it to 26 by dropping defender Owen Wijndal, midfielders Jordy Clasie and Marten de Roon and the striking pair of Arnaut Danjuma and Luuk de Jong.
Also cut from the preliminary selection was Rick Karsdorp, just two days after helping AS Roma win the Europa Conference League by beating Dutch club Feyenoord in Tirana.
Georginio Wijnaldum was the notable omission from preliminary squad as he has struggled for form at Paris St Germain.
Mexican-based striker Vincent Janssen, who was surprise selection a fortnight ago, will miss the first game as he is getting married on June 5.
The former Tottenham Hotspur forward last played for Holland in 2017.
“Everyone knows that the agenda is already dominated by the coming World Cup. Soon there will be little time to prepare, so we must make the most of the available time,” said van Gaal in a statement released by the Dutch Football Association.
“Only during this period and in September we still have the chance to see new players at work. That is why I now choose Bruno Martins Indi, Jerdy Schouten and Vincent Janssen.
“These are interesting players that I would like to see up close during a training camp, although I already know some of them of course. I would like to emphasise that everyone remains in the picture with us, including the players who have not received an invitation now,” he added.
Squad:
Goalkeepers: Jasper Cillessen (Valencia), Mark Flekken (SC Freiburg), Tim Krul (Norwich City)
Defenders: Nathan Ake (Manchester City), Daley Blind (Ajax Amsterdam), Matthijs de Ligt (Juventus), Stefan de Vrij, Denzel Dumfries (both Inter Milan), Hans Hateboer (Atalanta), Tyrell Malacia (Feyenoord Rotterdam), Bruno Martins Indi (AZ Alkmaar), Jordan Teze (PSV Eindhoven), Jurrien Timber (Ajax Amsterdam), Virgil van Dijk (Liverpool)
Midfielders: Frenkie de Jong (Barcelona), Davy Klaassen (Ajax Amsterdam), Teun Koopmeiners (Atalanta), Jerdy Schouten (Bologna), Guus Til (Feyenoord Rotterdam)
Forwards: Steven Berghuis (Ajax Amsterdam), Steven Bergwijn (Tottenham Hotspur), Memphis Depay (Barcelona), Cody Gakpo (PSV Eindhoven), Vincent Janssen (Monterrey), Noa Lang (Club Brugge), Wout Weghorst (Burnley)