Ahoy,
For the GTK 4 version only, I'm finding on Trixie that gcr-viewer terminates with SIGSEGV when one attempts to open a file that contains a private key, except possibly for PKCS#12 files—only because I think viewing all PKCS#12 files is broken in gcr-viewer-gtk4 for an unrelated reason (different issue) and thus can't test with one. This crash happens before any user interface is displayed just after the process starts.

It is a fortunate coincidence that an installed test in glib-networking-tests works well to demonstrate the problem:

(gdb) gcr-viewer-gtk4 /usr/libexec/installed-tests/glib-networking/files/client-and-key.pem
Thread 1 "gcr-viewer-gtk4" received signal SIGSEGV, Segmentation fault.
on_parser_parsed (parser=<optimized out>, user_data=0x7fffffffd500) at ../tools/viewer/viewer.c:46
46		*cert = gcr_simple_certificate_new (attr->value, attr->length);

Apparently, attr—as returned by gck_attributes_find() just prior—is NULL here. The Gck 4 docs say this is legitimate if the attribute isn't found, so it's wrong for Gcr to not check for this here.

#0  on_parser_parsed (parser=<optimized out>, user_data=0x7fffffffd530) at ../tools/viewer/viewer.c:46
#4  0x00007ffff7167663 in <emit signal 'parsed' on instance 0x55555575bb00 [GcrParser]> (instance=instance@entry=0x55555575bb00, signal_id=<optimized out>, detail=detail@entry=0) at ../../../gobject/gsignal.c:3597
    #1  0x00007ffff714bb81 in _g_closure_invoke_va (closure=closure@entry=0x555555763d70, return_value=return_value@entry=0x0, instance=instance@entry=0x55555575bb00, args=args@entry=0x7fffffffd230, n_params=n_params@entry=0, param_types=param_types@entry=0x0) at ../../../gobject/gclosure.c:898
    #2  0x00007ffff71618b8 in signal_emit_valist_unlocked (instance=instance@entry=0x55555575bb00, signal_id=signal_id@entry=126, detail=detail@entry=0, var_args=var_args@entry=0x7fffffffd230) at ../../../gobject/gsignal.c:3438
    #3  0x00007ffff71675a6 in g_signal_emit_valist (instance=0x55555575bb00, signal_id=126, detail=0, var_args=0x7fffffffd230) at ../../../gobject/gsignal.c:3277
#5  0x00007ffff7f076f0 in _gcr_parser_fire_parsed (self=self@entry=0x55555575bb00 [GcrParser], parsed=parsed@entry=0x55555569efe0) at ../gcr/gcr-parser.c:484
#6  0x00007ffff7ef46d6 in _gcr_parser_parse_der_private_key_rsa (self=0x55555575bb00 [GcrParser], data=0x5555556bc800) at ../gcr/gcr-parser-libgcrypt.c:80
#7  0x00007ffff7f08329 in handle_pem_data (type=type@entry=1128, data=data@entry=0x5555556bc800, outer=outer@entry=0x55555569f640, headers=headers@entry=0x0, user_data=user_data@entry=0x7fffffffd470) at ../gcr/gcr-parser.c:1221
#8  0x00007ffff7f16fb7 in egg_armor_parse (data=data@entry=0x555555763c10, callback=callback@entry=0x7ffff7f081c0 <handle_pem_data>, user_data=user_data@entry=0x7fffffffd470) at ../egg/egg-armor.c:316
#9  0x00007ffff7f067f2 in handle_pem_format (self=<optimized out>, subformat=0, data=0x555555763c10) at ../gcr/gcr-parser.c:1245
#10 parse_pem (self=<optimized out>, data=0x555555763c10) at ../gcr/gcr-parser.c:1258
#11 0x00007ffff7f06bf2 in parser_format_foreach (key=key@entry=0x7ffff7f5be60 <parser_normal>, value=value@entry=0x7ffff7f5be60 <parser_normal>, data=data@entry=0x7fffffffd4c0) at ../gcr/gcr-parser.c:1516
#12 0x00007ffff7f07b1e in gcr_parser_parse_bytes (self=self@entry=0x55555575bb00 [GcrParser], data=data@entry=0x555555763c10, error=error@entry=0x7fffffffd528) at ../gcr/gcr-parser.c:1777
#13 0x0000555555558b45 in simple_certificate_new_from_file (cancellable=0x0, file=0x555555764800, error=0x7fffffffd528) at ../tools/viewer/viewer.c:69
#14 simple_certificate_new_from_commandline_arg (cancellable=0x0, arg=<optimized out>, error=0x7fffffffd528) at ../tools/viewer/viewer.c:94
#15 activate (app=<optimized out>, user_data=<optimized out>) at ../tools/viewer/viewer.c:127
#20 0x00007ffff7167663 in <emit signal 'activate' on instance 0x555555580280 [GtkApplication]> (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>) at ../../../gobject/gsignal.c:3597
    #16 0x00007ffff714b950 in g_closure_invoke (closure=0x555555582390, return_value=0x0, n_param_values=n_param_values@entry=1, param_values=param_values@entry=0x7fffffffd750, invocation_hint=invocation_hint@entry=0x7fffffffd6a0) at ../../../gobject/gclosure.c:835
    #17 0x00007ffff715fd43 in signal_emit_unlocked_R (node=node@entry=0x7fffffffd810, detail=detail@entry=0, instance=instance@entry=0x555555580280, emission_return=emission_return@entry=0x0, instance_and_params=instance_and_params@entry=0x7fffffffd750) at ../../../gobject/gsignal.c:3902
    #18 0x00007ffff7161769 in signal_emit_valist_unlocked (instance=instance@entry=0x555555580280, signal_id=signal_id@entry=8, detail=detail@entry=0, var_args=var_args@entry=0x7fffffffd970) at ../../../gobject/gsignal.c:3534
    #19 0x00007ffff71675a6 in g_signal_emit_valist (instance=0x555555580280, signal_id=8, detail=0, var_args=0x7fffffffd970) at ../../../gobject/gsignal.c:3277
#21 0x00007ffff7281ca0 in g_application_real_local_command_line (application=0x555555580280 [GtkApplication], arguments=0x7fffffffdac8, exit_status=0x7fffffffdac4) at ../../../gio/gapplication.c:1188
#22 0x00007ffff7281e38 in g_application_run (application=application@entry=0x555555580280 [GtkApplication], argc=argc@entry=2, argv=argv@entry=0x7fffffffdc38) at ../../../gio/gapplication.c:2684
#23 0x0000555555557758 in main (argc=2, argv=0x7fffffffdc38) at ../tools/viewer/viewer.c:152

I'm curious why _gcr_parser_parse_der_private_key_rsa gets invoked even when all input data is PEM-encoded. Let me know if you can't reproduce; I have an esoteric PKCS#11 setup and maybe that influences Gck somehow.

Apparently I reported (almost) this same issue upstream a few months ago, but that was in regards to handling illegitimate data. Not being able to import private keys, apparently at all, is a much greater problem.