Is your feature request related to a problem? Please describe.
When I work on large projects with specific brushes and masks, sometimes I forget the mask I have on my brush. Also, let's say I want to change the mask (add or take away a set of blocks), as of right now, we have to completely rewrite the mask command or find it by tabbing into chat hundreds of lines to find the last one I used.

Describe the solution you'd like
The ideal solution I would like seen is a view flag (-v), an add flag (-a), and a remove flag (-r) for both the mask and gmask command so we can view and edit the current mask via commands. I feel liek the ideal solution would be a chat based GUI, which would be sent to the user and displayed like this:

----- Edit Mask -----
[-] Block1[property=value]
[-] Block2
[-] Block3
[ADD] [CLEAR]
----- <<< Page x of y >>> -----

The [-] signs would have a hover event saying "Remove from current mask" and when clicked, it would run the command /mask -r <block being removed>.

The [ADD] text would have a hover event saying "Add to current mask" and when clicked, it would suggest the command /mask -a allowing the user to add to their current mask without having to retype the whole thing, and allowing masks to be as large as the user wants due to effectively removing the limit of the chat size.

The [CLEAR] text would have a hover event saying "Clear the current mask" and when clicked, it would run the command /mask to clear it.

Describe alternatives you've considered
Alternatively, the -a and -r flags could be added and the -v flag would just send a list of the current materials in the mask rather than having the buttons to edit the mask like shown above.

your suggestion seems to assume that every mask is a BlockMask...
The mask interface itself really doesn't expose anything more than the test method, and given that third party plugins can even register masks, this isn't very extensible to mask combinations and other complex mask types.

That would complicate things now that I think about it more- however, simply something to view the current mask would be nice imo.