The passage is "If any of you put a stumbling block before one of these little ones who believe in me, it would be better for you if a great millstone were hung around your neck and you were thrown into the sea." (Mark 9:42)
Many of us think that rejection of gay kids is in fact putting a stumbling block before them. Certainly it can have serious negative consequences.
Ideally, the child will be mature enough to realize that their grandmother could be wrong but still be worth loving. But it's really the adult's responsibility to prevent the problem, per Jesus' saying.
[I looked up the Greek behind Mark 9:42, because translations are so different. What is translated "put a stumbling block before" is a single word. While that's the literal meaning, the meaning in context includes being the cause of various kinds of disaster, often metaphorical such as the cause of sin, the cause of falling away from Jesus, or causing offense. All of these occur in the NT. The context doesn't make the meaning entirely clear, but given the extreme consequences I think it suggests falling away from Jesus, not just committing a specific sin.]
His grandparents are not wrong, anyone professing they are wrong, is indeed professing that God is wrong.
I think the following verses should help clear it up for what God thinks on the subject, as written by men influenced and lead by His Holy Spirit:
“Do not be deceived; neither fornicators nor idolaters nor adulterers nor boy prostitutes nor sodomites nor thieves nor the greedy nor drunkards nor slanderers nor robbers will inherit the kingdom of God.”;
“...the understanding that law is meant not for a righteous person but for the lawless and unruly, the godless and sinful, the unholy and profane, those who kill their fathers or mothers, murderers, the unchaste, sodomites, kidnappers, liars, perjurers, and whatever else is opposed to sound teaching, according to the glorious gospel of the blessed God, with which I have been entrusted.“;
“...Likewise, Sodom, Gomorrah, and the surrounding towns, which, in the same manner as they, indulged in sexual promiscuity and practiced unnatural vice, serve as an example by undergoing a punishment of eternal fire.“;
“Their females exchanged natural relations for unnatural, and the males likewise gave up natural relations with females and burned with lust for one another. Males did shameful things with males and thus received in their own persons (bodies) the due penalty for their perversity.“
I think that’s enough said.