GOG uses ScummVM for their current version of Phantasmagoria.
It is listed on the GOG webpage -- "This game is powered by ScummVM"
https://www.gog.com/game/phantasmagoriaGOG has switched their DOS installers to ScummVM installers whenever the game their selling is supported by ScummVM, and ScummVM is continuing to add games to their list.
ScummVM should allow you to run full screen.
Are you using a disc version or a download version?
And if using a GOG version, is it a recent download or an older one from back when they used DOSBox for the game?
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If you have a disc version and want to try it with ScummVM, the files you need from the game discs are on this page:
https://wiki.scummvm.org/index.php/DatafilesAll SCI32 (SCI2/3) games
Copy all files from each disk or CD to the game directory. Some games need additional work:
Games with multiple CDs
Directories that exist on multiple discs (e.g. ROBOT and VMD directories) must be merged together, not replaced.
Rename the RESOURCE.SFX from each CD to RESSFX.00<cdnumber>, and the RESOURCE.AUD from each CD to RESAUD.00<cdnumber>, to match the corresponding RESSCI.00<cdnumber> file that exists on each CD.
It should be safe to replace any other files from later CDs that were copied already from earlier CDs, except in Phantasmagoria 1 (see below).
Phantasmagoria 1 from CDs
Copy the PDOCO.TXT from the first CD only.
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If you have the CD version of Phantasmagoria and would rather use DOSBox, check the installer from here
http://sierrahelp.com/Patches-Updates/NewSierraInstallers.htmlPhantasmagoria for DOSBox
http://sierrahelp.com/Files/NewInstallers/PhantasmagoriaForDOSBox.zip - Another installer that makes use of Enrico Rolfi's (Endroz) UniSCI tool (to enable CD-less play). It includes a wizard to convert the installed folder into a package ready to burn to DVD with AutoPlay that mimics the original AutoPlay, but initializes quickly. When installing from the created DVD, you can choose between full hard drive install (no disc required for play) or minimal install (DVD needed for play, but with no disc swapping). See this thread for all the features.
http://www.sierrahelp.com/forums/viewtopic.php?f=34&t=633