Hi,
I don't have any problems to flower this species. It grows besides my U. sandersonii. I keep the pot wet all year round. The plants are in my greenhouse, where it can get very hot (sometimes about 40°C) . I use the tray-method to water the plants. They usually stand in 1cm of water. Sometimes, there is even no water in the saucer, but i never let the pot dry out. Last year, the flowers set seed, without manual pollination. But a G. violacea flowered at the same time. So i can't exclude a cross-polination. I didn't manage to germinate the seeds. Interestingly, my G. repens has never flowered under the same conditions.
I don't think, it's an annual. If someone knows it better, please let me know.
Christian