Go with the gerund. The intuition for tag names is that they complete the sentence “this question is about ____”. Infinitives are a poor fit.
Looking at existing sites, a tag called align
exists only on TeXTeX, where it's a proper noun and not the verb; aligning
exists nowhere; alignment
exists on 10 sites. A tag called load
exists on many sites, but it's about the system load (so a noun), not about the verb (at least officially: it does get a lot of misuse, where “load” is used to mean “I happen to be loading something” instead of “this question is about loading something”).