diff --git a/dotcom-rendering/src/components/SignInGate/types.ts b/dotcom-rendering/src/components/SignInGate/types.ts
index be6bfe1703c..817b7e9ccd5 100644
--- a/dotcom-rendering/src/components/SignInGate/types.ts
+++ b/dotcom-rendering/src/components/SignInGate/types.ts
@@ -108,6 +108,8 @@ export interface AuxiaAPIResponseDataUserTreatment {
 	surface: string;
 }
 
+export type ShowGateValues = 'true' | 'mandatory' | 'dismissible' | undefined;
+
 export interface AuxiaProxyGetTreatmentsPayload {
 	browserId: string | undefined;
 	isSupporter: boolean;
@@ -123,7 +125,7 @@ export interface AuxiaProxyGetTreatmentsPayload {
 	should_show_legacy_gate_tmp: boolean; // [1]
 	hasConsented: boolean;
 	shouldServeDismissible: boolean; // [2]
-	mustShowDefaultGate: boolean; // [3]
+	showDefaultGate: ShowGateValues; // [3]
 }
 
 // [1]
@@ -146,19 +148,19 @@ export interface AuxiaProxyGetTreatmentsPayload {
 // We will be setting it to false.
 
 // [2]
-// date: 03rd July 2025
-// If shouldServeDismissible is true, we should show a dismissible gate.
+// date: 03rd July 2025// If shouldServeDismissible is true, we should show a dismissible gate.
 
 // [3]
-// date: 23rd July 2025
+
+// date: 25rd July 2025
 // author: Pascal
-// In order to facilitate internal testing, this attribute forces
-// the display of a sign-in gate, namely the default gu gate. If it is true then
-// the default gate is going to be displayed. Note that this applies to both auxia and
-// non auxia audiences. In particular because it also applies to auxia audiences, for which
-// the value of should_show_legacy_gate_tmp is ignored, then the information needs to come to
-// the SDC server as a new parameter in the GetTreatments payload. To trigger
-// gate display, the url should have query parameter `showgate=true`
+
+// In order to facilitate internal testing, this attribute, when defined, forces
+// the display of a sign-in gate. The values 'true' and 'dismissible' displays the
+// dismissible variant of the gu default gate, and the value 'mandatory' displays
+// the mandatory variant of the gu default gate.
+
+// Note that this attributes override the value of should_show_legacy_gate_tmp.
 
 export interface AuxiaProxyGetTreatmentsResponse {
 	status: boolean;
diff --git a/dotcom-rendering/src/components/SignInGateSelector.importable.tsx b/dotcom-rendering/src/components/SignInGateSelector.importable.tsx
index fd0f2a1a3be..3cdceab4825 100644
--- a/dotcom-rendering/src/components/SignInGateSelector.importable.tsx
+++ b/dotcom-rendering/src/components/SignInGateSelector.importable.tsx
@@ -32,6 +32,7 @@ import type {
 	AuxiaProxyLogTreatmentInteractionPayload,
 	CanShowGateProps,
 	CurrentSignInGateABTest,
+	ShowGateValues,
 } from './SignInGate/types';
 
 // ------------------------------------------------------------------------------------------
@@ -291,7 +292,7 @@ const fetchProxyGetTreatments = async (
 	should_show_legacy_gate_tmp: boolean,
 	hasConsented: boolean,
 	shouldServeDismissible: boolean,
-	mustShowDefaultGate: boolean,
+	showDefaultGate: ShowGateValues,
 ): Promise<AuxiaProxyGetTreatmentsResponse> => {
 	// pageId example: 'money/2017/mar/10/ministers-to-criminalise-use-of-ticket-tout-harvesting-software'
 	const articleIdentifier = `www.theguardian.com/${pageId}`;
@@ -315,7 +316,7 @@ const fetchProxyGetTreatments = async (
 		should_show_legacy_gate_tmp,
 		hasConsented,
 		shouldServeDismissible,
-		mustShowDefaultGate,
+		showDefaultGate,
 	};
 	const params = {
 		method: 'POST',
@@ -332,8 +333,8 @@ const fetchProxyGetTreatments = async (
 
 const decideShouldServeDismissible = (): boolean => {
 	// Return a boolean indicating whether or not we accept mandatory gates for this call.
-	// If the answer is `false` this doesn't decide whether the gate should be displayed or not,
-	// it only means that if a gate is returned, then it must be mandatory.
+	// If the answer is `true` this doesn't decide whether the gate should be displayed or not,
+	// it only means that if a gate is returned, then it must be dismissible (not be mandatory).
 
 	// Now the question is how do we decide the answer ?
 	// We return false if the following query parameter is present in the url:
@@ -348,7 +349,7 @@ const decideShouldServeDismissible = (): boolean => {
 	return false;
 };
 
-const decideMustShowDefaultGate = (): boolean => {
+const decideShowDefaultGate = (): ShowGateValues => {
 	// In order to facilitate internal testing, this function observes a query parameter which forces
 	// the display of a sign-in gate, namely the default gu gate. If this returns true then
 	// the default gate is going to be displayed. Note that this applies to both auxia and
@@ -360,7 +361,17 @@ const decideMustShowDefaultGate = (): boolean => {
 
 	const params = new URLSearchParams(window.location.search);
 	const value: string | null = params.get('showgate');
-	return value === 'true';
+
+	if (value === null) {
+		return undefined;
+	}
+
+	const validValues = ['true', 'dismissible', 'mandatory'];
+	if (validValues.includes(value)) {
+		return value as ShowGateValues;
+	}
+
+	return undefined;
 };
 
 const buildAuxiaGateDisplayData = async (
@@ -402,7 +413,7 @@ const buildAuxiaGateDisplayData = async (
 
 	const shouldServeDismissible = decideShouldServeDismissible();
 
-	const mustShowDefaultGate = decideMustShowDefaultGate();
+	const showDefaultGate = decideShowDefaultGate();
 
 	const response = await fetchProxyGetTreatments(
 		contributionsServiceUrl,
@@ -420,7 +431,7 @@ const buildAuxiaGateDisplayData = async (
 		should_show_legacy_gate_tmp,
 		readerPersonalData.hasConsented,
 		shouldServeDismissible,
-		mustShowDefaultGate,
+		showDefaultGate,
 	);
 
 	if (response.status && response.data) {