Are you sure? It looks to me like it uses replace except for the case of INCR (which isn't implemented in pgtkselect.c).

for (cs = frame->converted_selections; cs; cs = cs->next)
{
if (cs->property == None)
continue;
bytes_remaining = cs->size;
bytes_remaining *= cs->format >> 3;
if (bytes_remaining <= max_bytes)
{
/* Send all the data at once, with minimal handshaking. */
TRACE1 ("Sending all %"pD"d bytes", bytes_remaining);
XChangeProperty (display, window, cs->property,
cs->type, cs->format, PropModeReplace,
cs->data, cs->size);
}

________________________________
From: Po Lu <luangruo@yahoo.com>
Sent: 28 July 2022 01:39
To: David Welch <davidwelch158@hotmail.com>
Cc: 56792-done@debbugs.gnu.org <56792-done@debbugs.gnu.org>
Subject: Re: bug#56792: 29.0.50; Emacs crashes when selecting text if xfreerdp is also running

> Of interest, what's the reason for not using GDK_PROP_MODE_REPLACE
> when setting the property in response to a selection request?

No reason in particular, the code is just kept closer to xselect.c.