The gospel is good news because it establishes our acceptance by God on the basis of the finished work of Christ rather than our works. That work of Christ was not undertaken just to change our label (e.g. saved or unsaved), but to change our very natures by enabling our dead spirits to be quickened by the infusion of the Holy Spirit. Believers in Christ are actually born of the Spirit, and made new creatures thereby. Christianity, therefore, is not about status but substance: we who were dead in trespasses and sins have actually been made alive in Christ Jesus, eternally.
On the other hand, the law (including tithing) was put in place (see Galatians 3:19-25) to serve as a custodian over sinners who were merely given the promise of Christ's coming, not the substance. It was only meant to keep Israel from running completely amok while they waited for the Promised One. Now that Christ has come, the law has been superseded by the realization of actual fellowship with God through the indwelling of his Holy Spirit. Reverting to the custodial to attain blessing from (or standing with) God through works of the law is a fool's errand, capable, only, of proving that in our natural selves we are sinners.
When I was a kid in nursery school, all the kids had their own nap mat. When nap time came, we unrolled our mat, laid down quietly and caught a few zzzz's. Back then I was always rested, my class functioned well and was always in good order, and I got stickers for being a good boy! Life was good and I had no reason to worry about anything.
Things were simpler then and more blissful, at least according to my idealized recollection, when there was something that clearly told me what to do and rewarded me for doing it. Nowadays, my life tends to be more harried: I'm often tired, things get chaotic, only duty tells me what to do, there are obligations to meet and bills to pay, and no one gives me so much as an "attaboy" for behaving myself. I wonder what people would think if I shucked my adult freedom and responsibility and hired a babysitter to keep me in line and give me stickers when I was doing well and being a good guy?
Christians who revert to legal principles (such as tithing) and depend upon rules and regulations in order to attain blessing or to assure themselves of standing with God have done that very thing. At best they are childish and at worst they are alienated from Christ. That's not a minor issue, but strikes at the heart of what it means to be born again. Such action is every bit as preposterous as an adult trying to establish the simple bliss of childhood by putting themselves in the charge of a babysitter!
What this really comes down to whether or not it's appropriate for a Christian to claim that any objective, specific behavior (like tithing) serves as a basis for securing blessing from God. In truth, that is purely and simply a legalistic principle that has no place in understanding a believer's relationship to God. Under the auspices of the Gospel, all one has to do in order to get whatever he or she needs from God is to ask in Jesus name! But then, that requires the messiness of faith rather than a cut and dried formula that can be exercised braindead and worse, Spiritless.
I could go on, and on, and on dealing with the folly that's been perpetrated on this subject (e.g. seed sowing, 100 fold return, etc.) but right now, I just don't have the tithe.