diff --git a/modules/gptPreAuction.js b/modules/gptPreAuction.js
index 29b9257d325..7052e6f97ea 100644
--- a/modules/gptPreAuction.js
+++ b/modules/gptPreAuction.js
@@ -76,21 +76,25 @@ export const appendGptSlots = adUnits => {
     return acc;
   }, {});
 
+  const adUnitPaths = {};
+
   window.googletag.pubads().getSlots().forEach(slot => {
     const matchingAdUnitCode = find(Object.keys(adUnitMap), customGptSlotMatching
       ? customGptSlotMatching(slot)
       : isAdUnitCodeMatchingSlot(slot));
 
     if (matchingAdUnitCode) {
+      const path = adUnitPaths[matchingAdUnitCode] = slot.getAdUnitPath();
       const adserver = {
         name: 'gam',
-        adslot: sanitizeSlotPath(slot.getAdUnitPath())
+        adslot: sanitizeSlotPath(path)
       };
       adUnitMap[matchingAdUnitCode].forEach((adUnit) => {
         deepSetValue(adUnit, 'ortb2Imp.ext.data.adserver', Object.assign({}, adUnit.ortb2Imp?.ext?.data?.adserver, adserver));
       });
     }
   });
+  return adUnitPaths;
 };
 
 const sanitizeSlotPath = (path) => {
@@ -103,7 +107,7 @@ const sanitizeSlotPath = (path) => {
   return path;
 }
 
-const defaultPreAuction = (adUnit, adServerAdSlot) => {
+const defaultPreAuction = (adUnit, adServerAdSlot, adUnitPath) => {
   const context = adUnit.ortb2Imp.ext.data;
 
   // use pbadslot if supplied
@@ -117,7 +121,7 @@ const defaultPreAuction = (adUnit, adServerAdSlot) => {
   }
 
   // find all GPT slots with this name
-  var gptSlots = window.googletag.pubads().getSlots().filter(slot => slot.getAdUnitPath() === adServerAdSlot);
+  var gptSlots = window.googletag.pubads().getSlots().filter(slot => slot.getAdUnitPath() === adUnitPath);
 
   if (gptSlots.length === 0) {
     return; // should never happen
@@ -167,7 +171,7 @@ function warnDeprecation(adUnit) {
 }
 
 export const makeBidRequestsHook = (fn, adUnits, ...args) => {
-  appendGptSlots(adUnits);
+  const adUnitPaths = appendGptSlots(adUnits);
   const { useDefaultPreAuction, customPreAuction } = _currentConfig;
   adUnits.forEach(adUnit => {
     // init the ortb2Imp if not done yet
@@ -190,9 +194,9 @@ export const makeBidRequestsHook = (fn, adUnits, ...args) => {
       let adserverSlot = deepAccess(context, 'data.adserver.adslot');
       let result;
       if (customPreAuction) {
-        result = customPreAuction(adUnit, adserverSlot);
+        result = customPreAuction(adUnit, adserverSlot, adUnitPaths[adUnit.code]);
       } else if (useDefaultPreAuction) {
-        result = defaultPreAuction(adUnit, adserverSlot);
+        result = defaultPreAuction(adUnit, adserverSlot, adUnitPaths[adUnit.code]);
       }
       if (result) {
         context.gpid = context.data.pbadslot = result;
diff --git a/test/spec/modules/gptPreAuction_spec.js b/test/spec/modules/gptPreAuction_spec.js
index 88062f2b785..bc41c811e7d 100644
--- a/test/spec/modules/gptPreAuction_spec.js
+++ b/test/spec/modules/gptPreAuction_spec.js
@@ -209,6 +209,17 @@ describe('GPT pre-auction module', () => {
       expect(adUnit.ortb2Imp.ext.data.adserver).to.deep.equal({ name: 'gam', adslot: '/12345/slotCode2' });
     });
 
+    it('returns full ad unit path even if mcmEnabled is true', () => {
+      config.setConfig({ gptPreAuction: { enabled: true, mcmEnabled: true } });
+      window.googletag.pubads().setSlots([
+        makeSlot({ code: '/12345,21212/slot', divId: 'div1' }),
+      ]);
+      const adUnit = {code: '/12345,21212/slot'};
+      expect(appendGptSlots([adUnit])).to.eql({
+        '/12345,21212/slot': '/12345,21212/slot'
+      })
+    })
+
     it('will not trim child id if mcmEnabled is not set to true', () => {
       window.googletag.pubads().setSlots([
         makeSlot({ code: '/12345,21212/slotCode1', divId: 'div1' }),
@@ -459,6 +470,23 @@ describe('GPT pre-auction module', () => {
       expect(returnedAdUnits).to.deep.equal(expectedAdUnits);
     });
 
+    it('should pass full slot path to customPreAuction when mcmEnabled is true', () => {
+      const customPreAuction = sinon.stub();
+      config.setConfig({
+        gptPreAuction: {
+          enabled: true,
+          mcmEnabled: true,
+          customPreAuction
+        }
+      });
+      window.googletag.pubads().setSlots([
+        makeSlot({ code: '/12345,21212/slot', divId: 'div1' }),
+      ]);
+      const adUnit = {code: '/12345,21212/slot'};
+      makeBidRequestsHook(sinon.stub(), [adUnit]);
+      sinon.assert.calledWith(customPreAuction, adUnit, '/12345/slot', adUnit.code);
+    });
+
     it('should use useDefaultPreAuction logic', () => {
       config.setConfig({
         gptPreAuction: {
@@ -540,6 +568,16 @@ describe('GPT pre-auction module', () => {
       runMakeBidRequests(testAdUnits);
       expect(returnedAdUnits).to.deep.equal(expectedAdUnits);
     });
+
+    it('sets gpid when mcmEnabled: true', () => {
+      config.setConfig({ gptPreAuction: { enabled: true, mcmEnabled: true } });
+      window.googletag.pubads().setSlots([
+        makeSlot({ code: '/12345,21212/slot', divId: 'div1' }),
+      ]);
+      const adUnit = {code: '/12345,21212/slot'};
+      makeBidRequestsHook(sinon.stub(), [adUnit]);
+      expect(adUnit.ortb2Imp.ext.gpid).to.eql('/12345/slot');
+    });
   });
 
   describe('pps gpt config', () => {