Unfortunately I don't think there's a command for that..
Maybe you can find (or make) a small utility that copies the current UTC time (same as GMT*) to a text file or to the clipboard. Then you can use the 'Execute' action in a filter to run that utility, and use the %PUT or %CLIPBOARD macro command to copy the time from there into your message text.
*: GMT and UTC use the same current time in practice. Note that the United Kingdom is not on GMT all year -- it uses British Summer Time (BST), which is one hour ahead of GMT, during the summer months.
I volunteer as a moderator to help keep the forum tidy. I do not work for Ritlabs SRL.