The Galatians were “taught in the word” by their teachers (6:6), but weren’t communicating any money to them (cf. Phil.4:15) to repay them. God always wanted the spiritual leaders who teach the Word paid (Deut.24:8 cf.12:19), with money and other “good things” (Gal.6:6); i.e., other things of value (cf.Gen.45:22,23).
If a church can’t afford a fulltime pastor, Paul encouraged spiritual leaders to work an outside job (Acts 20:17,18,34). That differs from under the law. When God gave the Levites no land to farm or raise animals on (Deut.14:27), that showed He didn’t want them working outside the ministry.
But the Galatians lost their “blessedness” (Gal.4:15), i.e., their thankfulness, when they fell for the law, so they went from wanting to repay Paul with their eyes for introducing them to grace, to not wanting to repay teachers anything for teaching them more about grace.
If Paul was still addressing the “spiritual” Galatians here (cf. 6:1), they too may not have wanted to pay their teachers, for they were teaching the law! But Paul’s advice to them as well would be to pay their pastor, but to also communicate the “good thing” of the grace message to him (IITim.1:13, 14). To threaten to not pay him until he taught grace would be legalism!
The legalizers were saying, “If you tell a Christian he’s not under the law, he’ll mock God and sin.” Paul says, “God is not mocked” when we sin under grace, for we still have to reap what we sow (Gal.6:7). For instance, Paul says not to get drunk (Eph.5:18). If you get drunk every day, you’ll die of liver disease. But that won’t be God judging your disobedience, like He did under the law. That will be you reaping what you sowed, i.e., reaping the natural consequences of your sin. Under the law, God judged Miriam with something that was not the natural consequences of her sinful rebellion (Num.12:1,10).
“The flesh” (Gal.6:8) is the part of you that desires to sin (cf.Eph.2:3). Fulfilling those desires is to “sow” to the flesh (Gal.6:8). If you do, you’ll reap “corruption,” which means the decomposition of your body by natural means (cf.Mt.6:19). You’ll also reap the natural corruption of your Christian testimony and your service for the Lord. But you’ll reap all that “of the flesh” (Gal.6:8), not of God.
Sowing to the Spirit means not sowing to the flesh. It involves sowing godliness and service for the Lord instead. Reaping “life everlasting” means reaping the benefits of the everlasting life you already have. I Timothy 4:8 promises that you’ll reap a blessed life in this present life, just as sowing to the flesh reaps corruption in this life.
If you’re not reaping a blessed life, just wait. Reaping takes time, so Paul says to keep sowing to the Spirit (Gal.6:9). You won’t reap a great life by worldly standards, but plenty of unbelievers have miserable lives—and they don’t have the Lord to see them through it. Verse 9 is the flip side of verse 7. There Paul says that just because God’s not punishing us as He did the Jews under the law doesn’t mean you won’t reap the evil that you sow. Here he’s saying that just because God’s not rewarding obedience like He did under the law doesn’t mean you won’t reap the good that you sow.
If you do good to “all” men (Gal.6:10) and not just your teachers, even unsaved men, you’ll be like God (Mt.5:44, 45). God did that as a “witness” (Acts 14:15-17), and so should we, when we have “opportunity.” We should do good to the household of believers too, to thank them when they bless us, as the Galatians wanted to do for Paul (Gal.4: 15)—but not just when others bless us (cf.Lu.6:33). We should be proactive in doing good to other believers and do good to them before they can do good to us.