Be not deceived (mē planāsthe). Present passive imperative with mē, “stop being led astray” (planaō, common verb to wander, to lead astray as in Mat 24:4.).
God is not mocked (ou muktērizetai). This rare verb (common in lxx) occurs in Lysias. It comes from muktēr (nose) and means to turn the nose up at one. That is done towards God, but never without punishment, Paul means to say. In particular, he means “an evasion of his laws which men think to accomplish, but, in fact, cannot” (Burton).
Whatsoever a man soweth (ho ean speirēi anthrōpos). Indefinite relative clause with ean and the active subjunctive (either aorist or present, form same here). One of the most frequent of ancient proverbs (Job 4:8; Arist., Rhet. iii. 3). Already in 2Co 9:6. Same point in Mat 7:16; Mar 4:26.
That (touto). That very thing, not something different.