I think the idea of tp as a gateway language is more that the experience of learning one language makes learning the next easier, not that learning English, say, from a tp textbook would be easier than from a textbook in your L1. In fact, of course, tp is badly set up for that purpose, lacking a convenient vocabulary for just about anything important in English: tense, gender, agreement, parts of speech, passives, and so on forever. The best you could do is put up a number of tp sentences and reasonably literal and literate English translation. Since tp does not do reductions at all well, if at all, this would show how various sorts of English reductions work, and that is an enormous part of English. The unreduced stuff is pretty much the same but English runs its NPs backwards.