Well, first of all it is important to remember that the community has some problematic habits regarding the conversation about who the GMs were and what their powers were.
I'll take a minute to clear this up once and for all.
First, not every character wearing the "Gamemaster Outfit" was a Gamemaster. It sounds crazy, “the name is on the outfit” some may try to argue, but all Cipsoft Members can use it.
And even among Cipsoft employees there were some “Gamemasters” whose task included, among other things, managing volunteer Gamemasters. I believe that your question is not aimed at these, after all, as Cipsoft Members your powers may be unimaginable.
Another important aspect, clandestine servers plays a significant role in rumors about the powers of a Gamemaster. I will not dwell on the subject, but it is known that the moderators of these clandestine organizations are called GMs, CMs or, why not, GODs and everyone has (almost?) unrestricted powers. Nothing that should be taken seriously, however they still have significant influence on the community and could not fail to score.
Now, let's get to your question.
Were volunteer Gamemasters able to create items? No, they cannot. There is no doubt about it.
This was recently clarified by a former Brazilian GM who made a video in honor of the blue covers. The video is nostalgic and it has english caption. It is worth checking!
"Tibia Gamemasters - Um Tributo de Uma Ex-GM - ENGLISH CAPTION!" - Youtube
I don't know how to add video, click on the image or here to access the video on youtube
As an offshoot to this video, there are several interviews of her to various portals and fansites. Including PortalTibia (a Promoted Tibia Fansite) in which she says in all letters:
Lelehiah: Some people claim that GMs could "create" items, summon creatures, etc. What can you tell us about it?
LL (Leen LeonHart): We couldn't. Only Cip members could because the client was completely different.
Source: Entrevista com ex GM Incangel/Leen Leonhart (Interview with former GM Incangel / Leen Leonhart)