See the chart at:
the row labels e.g. 03bx are the first 3 characters you type after the ctrl+shift you will see below. The last “x” value is the column of the table
Ctrl Shift u
at the same time, then release all three keys and type the four-digit code and press Enter.
Examples of Greek characters in GIMP:
“mu” μ: Ctrl shift u 03bc Enter
“gamma” ϒ: Ctrl Shift u 03d2 Enter