I agree with everyone's sentiments. For larger vessels there are various other sources for information, such as Equasis and Miramar, but the amount of data available seems to decrease with vessel size. There really has been no alternative to Grosstonnage for researching many tugboats and fishing vessels. Some may be found on national databases, such as those maintained by the United States and Canada, but in several cases I've found those to be omitting IMO numbers that Grosstonnage had, especially for vessels toeing the 100 GT line. With enough digging they can be found on some of the AIS sites, but in most cases there is little other information attached.
Such a shame that all of that valuable information will be evaporating into thin air.