Bug 777589 - explains GIF animation's minimum framerate.

Though we display the frame delay as milliseconds, it is actually stored
as unsigned centiseconds in GIF. This means that displaying milliseconds
can be misleading since we round every value to tens and it also means
that 10ms is the lower delay allowed. This limitation is in the GIF
format.
Other animation formats may not have this limitation and we try and keep
consistent export UIs. Also the layer tagging for animation uses "ms"
syntax. So I just keep the delay entry as ms, but sets a lower allowed
value and makes it "snaps to ticks" (i.e. snaps to 10 ms increments).
Finally I add a tooltip to the field saying "GIF supports hundredths of
a second precision."
Hopefully this should make things clearer and not mislead people about
what the GIF format allows.

plug-ins/ui/plug-in-file-gif.ui

@@ -158,6 +158,7 @@
                         <property name="label" translatable="yes">_Delay between frames where unspecified:</property>
                         <property name="use_underline">True</property>
                         <property name="mnemonic_widget">delay-spin</property>
+                        <property name="tooltip-text" translatable="yes">GIF supports hundredths of a second precision.</property>
                       </object>
                       <packing>
                         <property name="expand">False</property>
@@ -173,6 +174,8 @@
                         <property name="shadow_type">none</property>
                         <property name="adjustment">delay</property>
                         <property name="climb_rate">1</property>
+                        <property name="snap-to-ticks">True</property>
+                        <property name="tooltip-text" translatable="yes">GIF supports hundredths of a second precision.</property>
                       </object>
                       <packing>
                         <property name="expand">False</property>
@@ -287,6 +290,7 @@
   </object>
   <object class="GtkAdjustment" id="delay">
     <property name="value">100</property>
+    <property name="lower">10</property>
     <property name="upper">65000</property>
     <property name="step_increment">10</property>
     <property name="page_increment">100</property>