diff --git a/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-decoder.any.js b/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-decoder.any.js
index c0221480da156..b7f1ca9c51e88 100644
--- a/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-decoder.any.js
+++ b/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-decoder.any.js
@@ -1,3 +1,5 @@
+// Additional tests can be found in ../gb18030/gb18030-decoder.any.js
+
 const gbkPointers = [
     6432, 7533, 7536, 7672, 7673, 7674, 7675, 7676, 7677, 7678, 7679, 7680, 7681, 7682, 7683, 7684,
     23766, 23770, 23771, 23772, 23773, 23774, 23776, 23777, 23778, 23779, 23780, 23781, 23782, 23784, 23785, 23786,
diff --git a/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-encoder-expected.txt b/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-encoder-expected.txt
index c1f1886d7e586..09997b714d666 100644
--- a/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-encoder-expected.txt
+++ b/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-encoder-expected.txt
@@ -11,4 +11,41 @@ PASS gbk encoder: legacy ICU special case 3
 PASS gbk encoder: legacy WebKit case 1
 PASS gbk encoder: legacy WebKit case 2
 PASS gbk encoder: legacy WebKit case 3
+PASS gbk encoder: U+10FFFF
+PASS gbk encoder: GB18030-2022 1
+PASS gbk encoder: GB18030-2022 2
+PASS gbk encoder: GB18030-2022 3
+PASS gbk encoder: GB18030-2022 4
+PASS gbk encoder: GB18030-2022 5
+PASS gbk encoder: GB18030-2022 6
+PASS gbk encoder: GB18030-2022 7
+PASS gbk encoder: GB18030-2022 8
+PASS gbk encoder: GB18030-2022 9
+PASS gbk encoder: GB18030-2022 10
+PASS gbk encoder: GB18030-2022 11
+PASS gbk encoder: GB18030-2022 12
+PASS gbk encoder: GB18030-2022 13
+PASS gbk encoder: GB18030-2022 14
+PASS gbk encoder: GB18030-2022 15
+PASS gbk encoder: GB18030-2022 16
+PASS gbk encoder: GB18030-2022 17
+PASS gbk encoder: GB18030-2022 18
+PASS gbk encoder: GB18030-2022 19
+PASS gbk encoder: GB18030-2022 20
+PASS gbk encoder: GB18030-2022 21
+PASS gbk encoder: GB18030-2022 22
+PASS gbk encoder: GB18030-2022 23
+PASS gbk encoder: GB18030-2022 24
+PASS gbk encoder: GB18030-2022 25
+PASS gbk encoder: GB18030-2022 26
+PASS gbk encoder: GB18030-2022 27
+PASS gbk encoder: GB18030-2022 28
+PASS gbk encoder: GB18030-2022 29
+PASS gbk encoder: GB18030-2022 30
+PASS gbk encoder: GB18030-2022 31
+PASS gbk encoder: GB18030-2022 32
+PASS gbk encoder: GB18030-2022 33
+PASS gbk encoder: GB18030-2022 34
+PASS gbk encoder: GB18030-2022 35
+PASS gbk encoder: GB18030-2022 36
 
diff --git a/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-encoder.html b/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-encoder.html
index e43cb73fea72e..11557242e3dc6 100644
--- a/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-encoder.html
+++ b/LayoutTests/imported/w3c/web-platform-tests/encoding/legacy-mb-schinese/gbk/gbk-encoder.html
@@ -23,4 +23,43 @@
  encode("\u00A5", "%26%23165%3B", "legacy WebKit case 1");
  encode("\u22EF", "%26%238943%3B", "legacy WebKit case 2");
  encode("\u301C", "%26%2312316%3B", "legacy WebKit case 3");
+ encode("\u{10FFFF}", "%26%231114111%3B", "U+10FFFF");
+
+  // GB18030-2022
+ encode("\uFE10", "%A6%D9", "GB18030-2022 1");
+ encode("\uFE12", "%A6%DA", "GB18030-2022 2");
+ encode("\uFE11", "%A6%DB", "GB18030-2022 3");
+ encode("\uFE13", "%A6%DC", "GB18030-2022 4");
+ encode("\uFE14", "%A6%DD", "GB18030-2022 5");
+ encode("\uFE15", "%A6%DE", "GB18030-2022 6");
+ encode("\uFE16", "%A6%DF", "GB18030-2022 7");
+ encode("\uFE17", "%A6%EC", "GB18030-2022 8");
+ encode("\uFE18", "%A6%ED", "GB18030-2022 9");
+ encode("\uFE19", "%A6%F3", "GB18030-2022 10");
+ encode("\u9FB4", "%FEY", "GB18030-2022 11");
+ encode("\u9FB5", "%FEa", "GB18030-2022 12");
+ encode("\u9FB6", "%FEf", "GB18030-2022 13");
+ encode("\u9FB7", "%FEg", "GB18030-2022 14");
+ encode("\u9FB8", "%FEm", "GB18030-2022 15");
+ encode("\u9FB9", "%FE~", "GB18030-2022 16");
+ encode("\u9FBA", "%FE%90", "GB18030-2022 17");
+ encode("\u9FBB", "%FE%A0", "GB18030-2022 18");
+ encode("\uE78D", "%A6%D9", "GB18030-2022 19");
+ encode("\uE78E", "%A6%DA", "GB18030-2022 20");
+ encode("\uE78F", "%A6%DB", "GB18030-2022 21");
+ encode("\uE790", "%A6%DC", "GB18030-2022 22");
+ encode("\uE791", "%A6%DD", "GB18030-2022 23");
+ encode("\uE792", "%A6%DE", "GB18030-2022 24");
+ encode("\uE793", "%A6%DF", "GB18030-2022 25");
+ encode("\uE794", "%A6%EC", "GB18030-2022 26");
+ encode("\uE795", "%A6%ED", "GB18030-2022 27");
+ encode("\uE796", "%A6%F3", "GB18030-2022 28");
+ encode("\uE81E", "%FEY", "GB18030-2022 29");
+ encode("\uE826", "%FEa", "GB18030-2022 30");
+ encode("\uE82B", "%FEf", "GB18030-2022 31");
+ encode("\uE82C", "%FEg", "GB18030-2022 32");
+ encode("\uE832", "%FEm", "GB18030-2022 33");
+ encode("\uE843", "%FE~", "GB18030-2022 34");
+ encode("\uE854", "%FE%90", "GB18030-2022 35");
+ encode("\uE864", "%FE%A0", "GB18030-2022 36");
 </script>
diff --git a/Source/WebCore/PAL/pal/text/TextCodecCJK.cpp b/Source/WebCore/PAL/pal/text/TextCodecCJK.cpp
index 44a716a4b5952..c03283d1ee318 100644
--- a/Source/WebCore/PAL/pal/text/TextCodecCJK.cpp
+++ b/Source/WebCore/PAL/pal/text/TextCodecCJK.cpp
@@ -899,7 +899,7 @@ static const GB18030EncodeIndex& gb18030EncodeIndex()
 // https://unicode-org.atlassian.net/browse/ICU-22357
 // The 2-byte values are handled correctly by values from gb18030()
 // but these need to be exceptions from gb18030Ranges().
-static std::optional<uint32_t> gb18030AsymmetricEncode(char32_t codePoint)
+static std::optional<uint16_t> gb18030AsymmetricEncode(UChar codePoint)
 {
     switch (codePoint) {
     case 0xE81E: return 0xFE59;
@@ -1031,12 +1031,11 @@ static Vector<uint8_t> gbEncodeShared(StringView string, Function<void(char32_t,
             unencodableHandler(codePoint, result);
             continue;
         }
-        if (isGBK == IsGBK::Yes) {
-            if (codePoint == 0x20AC) {
-                result.append(0x80);
-                continue;
-            }
-        } else if (auto encoded = gb18030AsymmetricEncode(codePoint)) {
+        if (isGBK == IsGBK::Yes && codePoint == 0x20AC) {
+            result.append(0x80);
+            continue;
+        }
+        if (auto encoded = gb18030AsymmetricEncode(codePoint)) {
             result.append(*encoded >> 8);
             result.append(*encoded);
             continue;