Creating GCs
To create a GC, call the new_gc() method of a drawable. This method can also take an associative array specifying some initial values (only foreground, background, tile, stipple, and clip_mask are currently supported). The rest have to be set manually. 

 1: <?php
 2:   // create a default GC
 3:   $gc = $window->new_gc();
 4: 
 5:   or
 6: 
 7:   // create GC and set some fields
 8:   $gc = $window->new_gc(array('foreground' => $fg_color,
 9:                               'background' => $bg_color));
10:   $gc->function   = GDK_XOR;
11:   $gc->line_width = 3;
12:   $gc->cap_style  = GDK_CAP_ROUND;
13: ?>
Using GCs
Simply pass the GC to the drawing function that you need to use. Which GC fields are used for the drawing operation depends on the function. 

<?php gdk::draw_line($pixmap, $gc, 0, 0, 100, 100); ?>