> подскажите куда копать дальше
В документацию(обратить внимание на подчеркнутую строку):
char *gdImageStringFT(gdImagePtr im, int *brect, int fg, char *fontname, double ptsize, double angle, int x, int y, char *string) (FUNCTION)
RECOMMENDED. New in 1.8.4. gdImageStringFT draws text using the FreeType 2.x library.
gdImageStringFT draws a string of anti-aliased characters on the image using the FreeType library to render user-supplied TrueType fonts. We do not provide TrueType fonts (.ttf and .ttc files). Obtaining them is entirely up to you. The string is anti-aliased, meaning that there should be fewer "jaggies" visible. The fontname is the full pathname to a TrueType font file, or a font face name if the GDFONTPATH environment variable or the compiled-in DEFAULT_FONTPATH macro of gdft.c have been set intelligently. In the absence of a full path, the font face name may be presented with or without extension (2.0.26).
The null-terminated string argument is considered to be encoded via
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
the UTF_8 standard;
^^^^^^^^^^^^^^^^^^^^
also, HTML entities are supported, including decimal, hexadecimal, and named entities (2.0.26). Those who are passing ordinary ASCII strings may have difficulty with the & character unless encoded correctly as & but should have no other difficulties.