I came across a small issue when trying to use some namecheap domains with Gitlab pages, namely two stupid joke pages: burgerlover.us and fsdbutton.xyz. I couldn’t get the verification to happen after adding the TXT entries. One simple support issue led me to find the reason, which is namecheap appends the domain automatically to the field.
So instead of entering _gitlab-pages-verification-code.www.burgerlover.us like gitlab suggests for the host, just enter _gitlab-pages-verification-code.www for example, which will fill in the rest. That did the trick.
Hope it helps some other folks.