GDPR is not a regulation for marketing but about how you collect and handle personally identifiable information (PII) during it's whole life cycle. This question is actually about the lawfulness of processing (Art. 6 GDPR).
You delimit your problem quite reasonably with this key phrase:
alert users of updates ... in relation to what they subscribed for
The subscription can seen as giving consent, fulfilling the condition 1a in the Art. 6.
Processing shall be lawful only if and to the extent that at least
one of the following applies:
a. the data subject has given consent to
the processing of his or her personal data for one or more specific
If your users explicitly subscribe to get these notifications and they can as easily unsubscribe, there shouldn't be any major problem with this. You could also add a declaration that by subscribing you allow the use of your email address for this purpose.
Keep your data processing well documented and transparent. In this case it means that as long as your users knows for what they are giving consent to and you limit your data processing only to that, this kind of functionality should not per se be a problem.