This 'rumor' comes from the camp of Nonoy. And this is a campaign strategy. The Liberal's after noticing that the gap on survey results is narrowing, try to pull down Villar with associating him to GMA. we know well that those who are associated with GMA are likely disliked by voters. It is clear that there is an attempt to spin us.
Now, the reason why GMA don't directly endorse Gibo is because of what I've mentioned above. I don't discount Manny Pacquiao but his transfer to Nacionalista doesn't have much impact to national political scene. If you have noticed, Visayas governors and Mayors (some are even in opposition) signify (signed) their support to Gibo. It was not clear though if they are also supporting Manzano and others but I think not. Only Gibo. In Ilocos, Gibo also got the support of local leaders.
Back to Villar. With his money, he don't need a support from unpopular president. Nor pray for it.
(Just an opinion)
Yes you are right these rumors could be from Noynoy camp since ang biggest threat (sa surveys) nya c Villar man.
Naa sad kai point why GMA never supports Gibo directly because para dli ma link c Gibo ni GMA.