Blah, pesky Italian machines & their electrics. Maybe there's some generic indicator problem with the Gilera which one of the other contributors can help you with, I'm not sure.
You say they work okay when you're stopped, have you tried fitting a new indicator relay/flasher unit? Either that or you may have a bad earth or an exposed wire somewhere along the wiring loom which is hitting off the frame & stopping power getting to the indicators.
When you're driving along the problem could be made worse from the engine vibrations & having to turn corners pulling the loom towards the frame.
Steo
November 2009