There are in fact two ways to deal with the problem of not being able to interact with notices on mastodon servers from gs.net. One is the already mentioned way of copying the notice's url into the search box of gs.net and, if it shows up, interact with it from there.

The other one is a kind of dirty workaround:

Create an account on a mastodon server. Subscribe to it from your account in gs.net. If a notice on a mastodon server you want to interact with cannot be found via its url in the search box of gs.net, reply to it with your mastodon account. As you're subscribed to your mastodon account from your account on gs.net, the mastodon notice and your mastodon reply will occur both in your gs.net Home timeline. Interact from gs.net with the mastodon notice (boost, like, reply). You may then delete your mastodon (!) reply to the mastodon notice. Your gs.net interaction ensures the mastodon notice stays in gs.net. Plus the mastodon account is now accessible in gs.net.

!gnusocial