"It has been my experience that WD 40 will not gum. I have never seen it happen in 35 + years of using it."
While I, on the other hand, HAVE seen it leave a very nasty, very gummy, and very hard to remove residue.
One of the people who used to work for my Father thought it would be a good idea to spray the pivots and mounts on one of his transits with WD-40 after they got caught in a sudden rain.
Around the same time Dad got a total station, and the transit was shelved. About 18 months later we had to use it again. Or, I should say, we tried to use it again.
I got the duty of cleaning it. The WD-40 had worked its way into the base plate and made it virtually impossible to turn the thing. It took me the better part of a week to fully disassemble it, clean it, put it back together, and run a series of calibration checks on it to make sure that it was still accurate.
I've also seen similar residue left on guns that have been hosed with WD-40 and then been put into long-term storage.
I simply won't use WD-40 on my guns anymore. It's not worth the hassle.