A possible soluton could be to use your GWT to set variables like start and filter as "useless", so Google will automatically ignore them.

You could also do this with Tag Meta. Create a rule for each gallery URL:

URL: /gallery/

And use {routeurl var1,var2,var3=myvalue,..,varN} macro to build the routing URL, using the "drop" variable feature (I just notice I forget to add in the documentation, I will add in the next version and in online documentation ASAP). You can ignore some variables just adding ! at the beginning, so in your case use:

{routeurl !start,!filter}

This should work...

Kind regards,
Luigi

No,
I wrote but probably not clear. If you need for ALL gallery, and they all have the common URL pattern to match like in your case, then you just need a single rule:

Last thing, remember to put this rule at the beginning (before others) and set "Last rule" to false, so it will be always applied to any /gallery/ URL and you can have additional rules for the same URL.

Kind regards,
Luigi

The canonical should be automatically generated if you put this macro in the canonical field:

Just try and check the generated pages.