Think for a second. This is taken directly from the abstract:
"We estimated that in 2011, suicide resulted in 46 deaths per 100,000 gay and bisexual men, while HIV resulted in 30 deaths per 100,000. Based on these estimates, suicide surpassed HIV as a leading cause of premature mortality for gay and bisexual men in 2007"
So to correct your statement. First of all, it's an estimation based on 7 year old figures. More importantly, it clearly says A leading cause, not THE leading cause. Look at the numbers. 46 deaths per 100.000 were suicide by their estimations. Think for a second. 46 per capita, or 100.000. In the US, 844 people died per 100.000 (1). So that would make 46 / 844 * 100 = 5,5%. This would make it at the level of strokes (5,1%), accidents (5,2%), respiratory disease (5,6%) and several times less than cancer (22%) or heart disease (23%) (2). It's still way too high, and gay men commit suicide at about twice the rate of the population average for males. It's not the leading cause though.
1:
https://www.cdc.gov/nchs/fastats/deaths.htm
2:
https://www.medicalnewstoday.com/articles/282929.php
He wasn't. Do yourself a favour and examine, or at least read the abstract of a study when you are presented with it.