What's the big deal any way? They are just two different names for the same thing, aren't they?
A resounding "No!" will be your response from the readers of gay fiction. While readers and authors of m/m romances often mistakenly use the terms interchangeably. Only the gay lit reader or author seems to know there is a difference.
And this is where the trouble begins. Gay literature enthusiasts are looking for a specific genre. They aren't interested in sweet little HEA stories. They want something that reflects the reality of their lives and don't care to read something they feel is an artificial construct catering to the sensibilities of a female audience. These readers can be very vocal in their criticism of m/m romance. Not to be outdone, m/m readers can likewise be critical of a perceived romance that isn't very romantic.
Gay literature is rarely romantic. It tends to be edgier, raw. The protagonists are men looking for a hook up. Sometimes it turns into more, but that is not a prerequisite. Sex does not equal love in this genre. Word choices are also very different. Words and phrases get used that would never find their way into m/m fiction.
It all comes down to expectations. The reader was expecting one genre and got the other. The gay lit reader knows what he wants, but a m/m story has been incorrectly labeled as a gay romance or thriller, etc. The m/m reader thinks gay lit is just another term for m/m romance.
And the flame war is on. Women are ruining gay lit. Men think they are the only ones who should write anything m/m. Why are are things drawn along gender lines? Because each gender has different expectations, wants and needs. That is not to say that members from each group can't cross the line in the sand or on the paper.
I know men who read and write m/m fiction. I know women who prefer to read gay lit, but can rarely write it themselves. I know authors whose pen names are different genders depending on what they are writing. It's all in the perception.
Readers and writers alike would be happier, if everyone understood the difference in the genres. Just a look at some of the reviews on Goodreads should convince people of this. Reviewers who say, "I would have given this a higher rating, but..." Either the story was an unrealistic portrayal of men, or it was just sex without any redeeming qualities, ie romance.
I am an author of m/m fiction. I will likely never be in a position where I can claim to be an author of gay lit. So you gentleman who like a nice romance are more than welcome to read my stories. For the men who prefer gay lit, I welcome your input, but I won't be offended if you don't care for my fluffy sweet little bit of fantasy. I am capable of writing edgier stories, but my female audience wanted more dialog and felt that I was being crude. One of my male readers asked if I had been a man in a previous life.
However you will never find the shy blushing virgin in any of my stories and no one ruins the afterglow by going to get a wet washcloth only to lob it back to the bathroom where it makes an unglamorous splat. I don't know any men who do that in real life. Roll over and go to sleep? Decide they must eat something right this second? Yes, but none of them were ever the fastidious type. So ladies, don't expect the men in m/m to act in a fashion that your own man won't.
It's all in the perceptions and expectations of your audience. We as writers should remember this and not confuse the reader with the incorrect label for our works.