diff --git a/man/repo-smartsync.1 b/man/repo-smartsync.1
index 8475adf..c1abbb3 100644
--- a/man/repo-smartsync.1
+++ b/man/repo-smartsync.1
@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
-.TH REPO "1" "August 2022" "repo smartsync" "Repo Manual"
+.TH REPO "1" "November 2022" "repo smartsync" "Repo Manual"
 .SH NAME
 repo \- repo smartsync - manual page for repo smartsync
 .SH SYNOPSIS
@@ -105,6 +105,13 @@
 .TP
 \fB\-\-no\-prune\fR
 do not delete refs that no longer exist on the remote
+.TP
+\fB\-\-auto\-gc\fR
+run garbage collection on all synced projects
+.TP
+\fB\-\-no\-auto\-gc\fR
+do not run garbage collection on any projects
+(default)
 .SS Logging options:
 .TP
 \fB\-v\fR, \fB\-\-verbose\fR
diff --git a/man/repo-sync.1 b/man/repo-sync.1
index 9cc528d..f006c03 100644
--- a/man/repo-sync.1
+++ b/man/repo-sync.1
@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
-.TH REPO "1" "August 2022" "repo sync" "Repo Manual"
+.TH REPO "1" "November 2022" "repo sync" "Repo Manual"
 .SH NAME
 repo \- repo sync - manual page for repo sync
 .SH SYNOPSIS
@@ -106,6 +106,13 @@
 \fB\-\-no\-prune\fR
 do not delete refs that no longer exist on the remote
 .TP
+\fB\-\-auto\-gc\fR
+run garbage collection on all synced projects
+.TP
+\fB\-\-no\-auto\-gc\fR
+do not run garbage collection on any projects
+(default)
+.TP
 \fB\-s\fR, \fB\-\-smart\-sync\fR
 smart sync using manifest from the latest known good
 build
@@ -200,6 +207,9 @@
 The \fB\-\-prune\fR option can be used to remove any refs that no longer exist on the
 remote.
 .PP
+The \fB\-\-auto\-gc\fR option can be used to trigger garbage collection on all projects.
+By default, repo does not run garbage collection.
+.PP
 SSH Connections
 .PP
 If at least one project remote URL uses an SSH connection (ssh://, git+ssh://,
diff --git a/man/repo.1 b/man/repo.1
index bdb705d..e7368a8 100644
--- a/man/repo.1
+++ b/man/repo.1
@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
-.TH REPO "1" "July 2022" "repo" "Repo Manual"
+.TH REPO "1" "November 2022" "repo" "Repo Manual"
 .SH NAME
 repo \- repository management tool built on top of git
 .SH SYNOPSIS
@@ -25,6 +25,10 @@
 \fB\-\-trace\fR
 trace git command execution (REPO_TRACE=1)
 .TP
+\fB\-\-trace_to_stderr\fR
+trace outputs go to stderr in addition to
+\&.repo/TRACE_FILE
+.TP
 \fB\-\-trace\-python\fR
 trace python command execution
 .TP
diff --git a/subcmds/sync.py b/subcmds/sync.py
index 83c9ad3..b7542cc 100644
--- a/subcmds/sync.py
+++ b/subcmds/sync.py
@@ -200,6 +200,9 @@
 The --prune option can be used to remove any refs that no longer
 exist on the remote.
 
+The --auto-gc option can be used to trigger garbage collection on all
+projects. By default, repo does not run garbage collection.
+
 # SSH Connections
 
 If at least one project remote URL uses an SSH connection (ssh://,
@@ -309,6 +312,10 @@
                  help='delete refs that no longer exist on the remote (default)')
     p.add_option('--no-prune', dest='prune', action='store_false',
                  help='do not delete refs that no longer exist on the remote')
+    p.add_option('--auto-gc', action='store_true',
+                 help='run garbage collection on all synced projects')
+    p.add_option('--no-auto-gc', dest='auto_gc', action='store_false',
+                 help='do not run garbage collection on any projects (default)')
     if show_smart:
       p.add_option('-s', '--smart-sync',
                    dest='smart_sync', action='store_true',
@@ -829,7 +836,14 @@
         project.config.SetString('gc.pruneExpire', None)
 
   def _GCProjects(self, projects, opt, err_event):
-    pm = Progress('Garbage collecting', len(projects), delay=False, quiet=opt.quiet)
+    """Perform garbage collection.
+
+    If We are skipping garbage collection (opt.auto_gc not set), we still want
+    to potentially mark objects precious, so that `git gc` does not discard
+    shared objects.
+    """
+    pm = Progress(f'{"" if opt.auto_gc else "NOT "}Garbage collecting',
+                  len(projects), delay=False, quiet=opt.quiet)
     pm.update(inc=0, msg='prescan')
 
     tidy_dirs = {}
@@ -849,6 +863,10 @@
             project.bare_git,
         )
 
+    if not opt.auto_gc:
+      pm.end()
+      return
+
     jobs = opt.jobs
 
     gc_args = ['--auto']