I don't think you can actually remove the limit, the GridField depends on the GridFieldPaginator and the GridFieldPaginator doesn't seem to let you set the number of pages to 0 (zero) or otherwise remove the limit. For me, I'm not sure about removing the limit altogether - I wouldn't want to load thousands of records in a single GridField :(
But if you're sure the number won't grow that far, I guess there's nothing against setting the number of items to 100 or 400 or...
Thanks a lot, martimiz. This is what I've been looking for.
In my case it's unlikely to get more than 30 records, but for the usability it's better to see more records, otherwise you quickly overlook some records. Thanks again!