Horsetail has been around for millions of years, unlike other weeds.

So its going to take time to get rid of them. Do not pull the weeds
out as you will leave the root, but persitent digging them out finally
gets rid of them.

But Yes go for the Roundup gell.

I beleive horsetail has a silica outer layer ( a bit like sand or
glass ) which is why normall liquid weedkillers do not have any
effect.

I quoted a job to get rid of all the visible weeds in a rockery, no
problem a good dose of weedkiller, and then a final dig after they had
died back. Only to find that the rockery was covered in horsetail when
I went back to dig it over. The owner said that she had known about
the problem, which is why her husband had not touched it for 5 years.
So I have now advised her to use the 'Roundup Gell'