diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9f09d92..d511165 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,9 +13,13 @@
 
 ## Breaking Changes
 
+- [#1239](https://github.com/oauth2-proxy/oauth2-proxy/pull/1239) GitLab groups sent in the `X-Forwarded-Groups` header
+  to the upstream server will no longer be prefixed with `group:`
+
 ## Changes since v7.1.3
 
 - [#1337](https://github.com/oauth2-proxy/oauth2-proxy/pull/1337) Changing user field type to text when using htpasswd (@pburgisser)
+- [#1239](https://github.com/oauth2-proxy/oauth2-proxy/pull/1239) Base GitLab provider implementation on OIDCProvider (@NickMeves)
 - [#1276](https://github.com/oauth2-proxy/oauth2-proxy/pull/1276) Update crypto and switched to new github.com/golang-jwt/jwt (@JVecsei)
 - [#1264](https://github.com/oauth2-proxy/oauth2-proxy/pull/1264) Update go-oidc to v3 (@NickMeves)
 - [#1233](https://github.com/oauth2-proxy/oauth2-proxy/pull/1233) Extend email-domain validation with sub-domain capability (@morarucostel)
diff --git a/providers/gitlab.go b/providers/gitlab.go
index 84ceda8..af0bcda 100644
--- a/providers/gitlab.go
+++ b/providers/gitlab.go
@@ -15,6 +15,7 @@ import (
 const (
 	gitlabProviderName = "GitLab"
 	gitlabDefaultScope = "openid email"
+	gitlabUserClaim    = "nickname"
 )
 
 // GitLabProvider represents a GitLab based Identity Provider
@@ -29,6 +30,7 @@ var _ Provider = (*GitLabProvider)(nil)
 // NewGitLabProvider initiates a new GitLabProvider
 func NewGitLabProvider(p *ProviderData) *GitLabProvider {
 	p.ProviderName = gitlabProviderName
+	p.UserClaim = gitlabUserClaim
 	if p.Scope == "" {
 		p.Scope = gitlabDefaultScope
 	}
@@ -119,17 +121,24 @@ func (p *GitLabProvider) EnrichSession(ctx context.Context, s *sessions.SessionS
 		return fmt.Errorf("user email is not verified")
 	}
 
-	s.User = userinfo.Username
-	s.Email = userinfo.Email
-	s.Groups = append(s.Groups, userinfo.Groups...)
+	if userinfo.Nickname != "" {
+		s.User = userinfo.Nickname
+	}
+	if userinfo.Email != "" {
+		s.Email = userinfo.Email
+	}
+	if len(userinfo.Groups) > 0 {
+		s.Groups = userinfo.Groups
+	}
 
+	// Add projects as `project:blah` to s.Groups
 	p.addProjectsToSession(ctx, s)
 
 	return nil
 }
 
 type gitlabUserinfo struct {
-	Username      string   `json:"nickname"`
+	Nickname      string   `json:"nickname"`
 	Email         string   `json:"email"`
 	EmailVerified bool     `json:"email_verified"`
 	Groups        []string `json:"groups"`
diff --git a/providers/provider_data.go b/providers/provider_data.go
index ccd6e47..de2aae3 100644
--- a/providers/provider_data.go
+++ b/providers/provider_data.go
@@ -41,6 +41,7 @@ type ProviderData struct {
 
 	// Common OIDC options for any OIDC-based providers to consume
 	AllowUnverifiedEmail bool
+	UserClaim            string
 	EmailClaim           string
 	GroupsClaim          string
 	Verifier             *oidc.IDTokenVerifier
@@ -156,6 +157,17 @@ func (p *ProviderData) buildSessionFromClaims(idToken *oidc.IDToken) (*sessions.
 	ss.Email = claims.Email
 	ss.Groups = claims.Groups
 
+	// Allow specialized providers that embed OIDCProvider to control the User
+	// claim. Not exposed as a configuration flag to generic OIDC provider
+	// users (yet).
+	if p.UserClaim != "" {
+		user, ok := claims.raw[p.UserClaim].(string)
+		if !ok {
+			return nil, fmt.Errorf("unable to extract custom UserClaim (%s)", p.UserClaim)
+		}
+		ss.User = user
+	}
+
 	// TODO (@NickMeves) Deprecate for dynamic claim to session mapping
 	if pref, ok := claims.raw["preferred_username"].(string); ok {
 		ss.PreferredUsername = pref
diff --git a/providers/provider_data_test.go b/providers/provider_data_test.go
index 09a19de..2748416 100644
--- a/providers/provider_data_test.go
+++ b/providers/provider_data_test.go
@@ -211,6 +211,7 @@ func TestProviderData_buildSessionFromClaims(t *testing.T) {
 	testCases := map[string]struct {
 		IDToken         idTokenClaims
 		AllowUnverified bool
+		UserClaim       string
 		EmailClaim      string
 		GroupsClaim     string
 		ExpectedError   error
@@ -259,6 +260,27 @@ func TestProviderData_buildSessionFromClaims(t *testing.T) {
 				PreferredUsername: "Complex Claim",
 			},
 		},
+		"User Claim Switched": {
+			IDToken:         defaultIDToken,
+			AllowUnverified: true,
+			UserClaim:       "phone_number",
+			EmailClaim:      "email",
+			GroupsClaim:     "groups",
+			ExpectedSession: &sessions.SessionState{
+				User:              "+4798765432",
+				Email:             "janed@me.com",
+				Groups:            []string{"test:a", "test:b"},
+				PreferredUsername: "Jane Dobbs",
+			},
+		},
+		"User Claim Invalid": {
+			IDToken:         defaultIDToken,
+			AllowUnverified: true,
+			UserClaim:       "groups",
+			EmailClaim:      "email",
+			GroupsClaim:     "groups",
+			ExpectedError:   errors.New("unable to extract custom UserClaim (groups)"),
+		},
 		"Email Claim Switched": {
 			IDToken:         unverifiedIDToken,
 			AllowUnverified: true,
@@ -332,6 +354,7 @@ func TestProviderData_buildSessionFromClaims(t *testing.T) {
 				),
 			}
 			provider.AllowUnverifiedEmail = tc.AllowUnverified
+			provider.UserClaim = tc.UserClaim
 			provider.EmailClaim = tc.EmailClaim
 			provider.GroupsClaim = tc.GroupsClaim