See title? Can I ask a question about recommendations for, say, Gmail alternatives?
Can I ask for recommended websites? seems to deal with a similar issue, but it doesn't specifically discuss webmail.
The answer is: It depends on the wording of the question - What are you looking to replace of GMail?
GMail consists of several parts:
The basic rules for asking good questions of course still also apply.
Longer answer to the client part: A replacement for GMail that looks for (Web-)Apps that handle Mail for you would be on-topic. You would need to specify what requirements you have for it to apply. If none of the requirements is: "I want to have an address that is not bound to the provider." then Yahoo Mail, Hotmail, etc. would be valid answers, they too can connect to POP3 and IMAP so they can connect to the GMail Mailservers (when I used Yahoo for that ~10 years ago I needed to pay them for this, don't know how its today). They are not websites that let you look at stuff, they are webapps that lets you create stuff.
I wrote a set of criteria to distinguish the two that I would apply here as well, because the website of a Webmail provider is different from the Webapp he serves.
+is designed to be interacted with
+possibility to create content (your E-Mail)
-possibility to transport (own) content out of the site - Most Mailhosters don't allow this
-usually no ability to import content to manipulate it
+Your content is your own, other people cannot change it (not even moderators) 1
+Your content is private to you
+Given you had the program of the hoster yourself and would run it on your own server without any data from the original source, the program would still be useful.
So at 5/7 I would conclude web based clients for generic mail servers are webapps.
1 Technically, the owner of the database that hosts your data could, but its not meant that way.
Webmail engines — the software you run on the server — is on-topic.
Webmail providers — companies who run the server — are off-topic.
This is a case among others of Are hosting recommendation requests on-topic?