Thanks for reporting this. A protection has been now put to the internal Write() function of the GDML parser to prevent worker threads to execute writing and restrict this to only the master thread. The fix will be available in the next patch release.

Respected Dr. Cosmo,

Greetings! I am not using the Write() function of the parser, but only Read().

My detector construction is below:

#include "DetectorConstruction.hh"
#include <G4LogicalVolume.hh>
#include <G4PVPlacement.hh>
#include <G4NistManager.hh>
#include <G4SystemOfUnits.hh>
#include <G4VisAttributes.hh>
#include <G4Box.hh>
#include <G4Tubs.hh>
#include <G4Orb.hh>
#include <G4SDManager.hh>
#include <G4RotationMatrix.hh>
#include <G4Transform3D.hh>
#include <G4SubtractionSolid.hh>
#include <G4Polyhedra.hh>
#include <G4Colour.hh>
#include <G4Material.hh>
#include <G4RunManager.hh>
#include <G4MultiFunctionalDetector.hh>
#include <G4PSEnergyDeposit.hh>
#include <G4SDManager.hh>
#include <G4SDParticleWithEnergyFilter.hh>
#include <G4PVParameterised.hh>
#include <G4LogicalVolume.hh>
#include <G4PVPlacement.hh>
#include <G4NistManager.hh>
#include <G4SystemOfUnits.hh>
#include <G4VisAttributes.hh>
#include <G4Box.hh>
#include <G4Orb.hh>
#include <G4SDManager.hh>
#include "G4GlobalMagFieldMessenger.hh"
#include "G4AutoDelete.hh"
#include "G4GeometryTolerance.hh"
#include "G4GeometryManager.hh"
#include "G4UnionSolid.hh"
#include "G4GDMLParser.hh"
#include "G4PSEnergyDeposit.hh"
#include <G4TessellatedSolid.hh>

#include "G4UserLimits.hh"

using namespace std;


G4VPhysicalVolume* DetectorConstruction::Construct() 
{
  G4String GDMLsolid;
  G4GDMLParser parser;
  parser.Read("/homes/sanchitsharma/Geant4/X-D-MSND/CAD_RUNS/source/src/475.gdml");
  G4VPhysicalVolume* world = parser.GetWorldVolume(); //world volume
  return world;
}
void DetectorConstruction::ConstructSDandField()
{ G4GDMLParser parser;
  parser.Read("/homes/sanchitsharma/Geant4/X-D-MSND/CAD_RUNS/source/src/475.gdml");
  G4SDManager* SDman = G4SDManager::GetSDMpointer();
  
  G4String fltName,particleName;
  // Sensitive Detector Name
  G4String phantomSDname = "BM";
  //------------------------
  // MultiFunctionalDetector
  //------------------------
  //
  // Define MultiFunctionalDetector with name.
  G4MultiFunctionalDetector* BM = new G4MultiFunctionalDetector(phantomSDname);
  SDman->AddNewDetector( BM );                // Register SD to SDManager.
  parser.GetVolume("fingdml")->SetSensitiveDetector(BM);    // Assign SD to the logical volume.
  G4String psName;
  G4PSEnergyDeposit * scorer0 = new G4PSEnergyDeposit(psName="eDep");
  G4SDParticleFilter* PFilter = new G4SDParticleFilter(fltName="PFilter");
  PFilter->add(particleName="triton");
  scorer0->SetFilter(PFilter);
  BM->RegisterPrimitive(scorer0);
}

Hi Sanchit, the title of this ticket explicitly refers to "GDML export"...
I have verified that "importing" (i.e. reading) GDML descriptions does not reproduce thread contention or crashes, as this operation should be only executed while defining the geometry in either your main() at the beginning, or inside your Construct() method, where everything is operated by the master thread.
The code you attached shows an issue though, i.e. you are repeating the GDML import within ConstructSDandField(). Why??  This may explain the crash.