The bulk of the price increase happened in literally the last...

The bulk of the price increase happened in literally the last few minutes of trading. It being the last day of the financial year, I suspect a fund with a reasonable holding in GSS wanted to improve their performance numbers. If this is the case, it will drift off during the course of next week.
I do not have the impression that GSS is a company that leaks so I doubt that is the cause. But, it is anticipating good news in the course of calendar 2019, including receiving FDA approvals, so with half of the year now gone, that hoped for good news is becoming more imminent.
It is a small, genuinely globally focused and well managed stock. Despite its excellent last 5 minutes of trading in the 2019 financial year, the year ahead looks very promising.