Gerund form (Vて形) + は + いけない Gerund form (Vて形) + は + いけません Doing that, it cannot work. => You should not do that.
赤毛
This entry has a typo in it. Clearly the "形" is not part of the entry (unless it is completely silent. ) I suspect that the "形" in "Gerund form (Vて形)" was made part of the entry by mistake.
Kurt Schmucker
Technically 行けない is as ok as is いけない. But since it is part of a grammatical structure, the "directed walking" aspect of 行 is unpreferable, thus Hiragana-form is found more often.
adnim
...? て形 means te-form, so it doesn't seem to be a typo. Unless I'm missing something here.