First Yes .art files from Allegro are gerber files.
Second. To create a new gerber in Manufacture -> artwork right click at one entry and choose add, now give it Your wanted name eg silk-top (no funny chars here).
Then You take the colour palette and choose all off, and now select from refdes the silk top, from package geometry and from board geometry, press ok to return to artwork window, now at Your new line (silk-top) right click and choose "match display".
Thats it.
Little hint. When done in the artwork window, click select all, right click at one line (does not matter witch one). Now click at "save all selected", this will create a FILMSETUP.txt in Your working directory, use this as a template for further jobs.
Second hint. Don't know if it fixed, but earlier when doing changes in artwork window, then on leaving ever first task to do is to go to file menu and click at save, NOT at the icon and NOT save as, otherwise You might as we experience that some last changes in artwork window disappears :-(.