The phrase "it's not a Gospel issue" tends to be used to defend obviously false doctrines. A common example is Theistic Evolution. The Doctrine of Creation is not a Gospel issue. Jesus is not going to look at a Theistic Evolutionist on judgement day and say "I know you repented of your sins, believed in me, and got baptised in the name of the Father, Son and Holy Spirit, but you believe the earth is 4.5 billion years old, so I never knew you, off to Hell with you." But is the fact it is not a Gospel issue enough of a reason to ignore it, or even to take the wrong side of it? Answer: No!
The thing about the Bible is it wasn't written just for fun. It isn't just a book of opinions. It isn't a book of lucky guesses. It isn't an unintelligible mess of stuff God wants us to interpret our own way, or wait for someone else to interpret for us. The Bible is the inspired word of God, written by men guided by the Holy Spirit, and was designed to make men wise to salvation, to help us form correct doctrine, for reproof, for correction and for instruction in righteousness (2 Timothy 3:15-17; 2 Peter 1:20-21).
That being said, there is no excuse for dodging or disagreeing with it. The Bible says the Earth was created in 6 days, roughly 6,000 years ago. Man was created on the 6th day. Man sinned, God cursed the earth. Man's sin grew beyond God's level of grace, God flooded the entire Earth, killing everything that breathed through its nostrils, saving only those on the ark. Man decided to band together, God dispersed them by confusing their languages. These are all absolute facts that can be discovered by reading the completely unambiguous first 11 chapters of Genesis.
If God did not want us to believe these things, He would not have inspired Moses to write it, or the prophets and Apostles to refer back to it several times. Indeed, Jesus Himself, though it was not a Gospel issue, referred back to Genesis to prove His views on the doctrine of marriage (Matthew 19:1-10; Mark 10:1-12). Furthermore, Jesus' final marching orders to the Apostles were "Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all things that I have commanded you" (Matthew 28:16-20 (emphasis mine).
Jesus commanded observation of the Scriptures. Indeed, during His life, that meant only the Old Testament, the New Testament not having been written yet. By commanding observation to the Scriptures, even so far as saying that you cannot believe Moses without believing Him, Jesus implicitly condemned every Theistic Evolutionist who ever called Moses a bronze age goat herder.
But still, as important as it is, it's not a Gospel issue, so even if it is important, we won't be judged for having the wrong view of things, right? Let me introduce you to what is, in my opinion, one of the most terrifying passages in all of Scripture: Now if anyone builds on this foundation with gold, silver, precious stones, wood, hay, straw, each one’s work will become clear; for the Day will declare it, because it will be revealed by fire; and the fire will test each one’s work, of what sort it is. If anyone’s work which he has built on it endures, he will receive a reward. If anyone’s work is burned, he will suffer loss; but he himself will be saved, yet so as through fire. - 1 Corinthians 3:12-15 (NKJV)
As a Christian, I do not fear Hell. I confess with my mouth the Lord Jesus, and believe in my heart God raised Him from the dead, and therefore I am both saved (Romans 10:9) and fully aware of that fact (1 John 5:13). I know that I am going to Heaven. But what will Heaven be like for me? The aforementioned "terrifying passage" indicates that I can make it far better by working for it. If I "build on the foundation" with gold, silver and precious stones, that's going to survive the fire, I'll receive a reward. If I build with wood, hay or straw, I will survive the fire thanks to the cross, but my works won't, and thus I'll suffer loss.
Thus, to you Theistic Evolutionists, I have this warning: Evolution is straw, hay and wood. Every time you preach Evolution, you are building with straw. Every time you discourage a Creation ministry because "it's not a Gospel issue", you are building with hay. Every time you make some kind of absurd excuse for either position, you are building with wood. Step through that fire unscathed by believing the Gospel, but do not expect your works will be rewarded if you insist on defending such heresies as Theistic Evolution.
Although Theistic Evolution was the chosen example of this article, the general concept applies far more broadly. Any issue on which one may disagree without losing salvation is covered by this principle. There is room for error, but the error should be considered an elephant in the room. Ignoring it doesn't make it go away.