If you can replace the 'to' with 'in order to', you should use the infinitive.
There should be a number of supplementary measures in order to create open spaces. In order to create open spaces, there should be....
X is one small but important step in order to decrease... -- doesn't work, so it's the case of preposition + gerund.
X is an important step to... [step to WHAT?] ==> step to success, step to prosperity... In other words, a noun is required (and in this case it's a gerund functioning as a noun - 'to decreasing'.)
You can't ask a number of supplementary measures or better planning [to WHAT], can you? So, 'to creating' is wrong and requires the infinitive:
There should be a number of measures to create open spaces.
Well, I tried to be logical.
Let's see what others have to say.
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