It is not possible to import fonts into gCast but you can use any font you like in the html source code of a campaign by using <font> tags.
We don't recommend using 'fancy fonts' (i.e. those outside the standard included in gCast's font family drop down) as they may not be web compatible and also may not be displayed on a client's machine if they're not installed on that machine/device.
Of course, if you still want to proceed with using a preferred font, we recommend that you use the 'font-family' tag so that the device being used to read the email by the recipient knows that there are other fonts it can use in the event that it doesn't recognise the font you've used as the primary font.
The other alternative is to use images to capture the text in the font of choice and then using the image in the campaign - be careful with this method though as many email servers specifically look for image:text ratios for deliverability purposes.
You can read more about web safe fonts here