Hi, It will be OK. Just let it grow normally.Feed and water as you would normally.Trim it as needed.It will fill it self in. If it is a major gap get some garden twine and tie pieces of the plant in such a way that you can pull them together carefully filling in the gap.I am not sure what the shape of you rosemary is but if it is round you should not have a problem.
Good Luck!

If possible expose that part more to sunlight to promote leaf growth and be patient. Also if you carefully trim the leaf buds, it forces the plant to produce more leaves. You could encourage the surrounding area to leaf out more to cover up that space. You could also let the ball grow out larger and continue nipping off buds to encourage new leaf growth. Be patient. It takes time.

what do you mean by buds? Do you mean to finger pinch off the leaves vs just sheering it to shape?

By buds, I mean the young potential leaves or simply the tips off each stem. I normally use my fingers to pinch off these off because I have more control over the plants. Some prefer using a very sharp and pointed pair of scissor to do this. This method helps to promote new side 'stems' which normally begins as leaves. Each time you nip off the tips, (usually) 2 more stems begin growing off the sides. Let those stems grow for a bit more, and the nip off the leaf buds on each one, 2 more stems get produced, let those grow and then nip the buds off once more... and so on... By this method, 1 branch can eventually become many branches... It will take time.

As for shaping the entire plant, I would use a pair of shears, but for the area surrounding the gap, I would suggest using more precise method of carefully nipping off buds off certain branches/stems to manipulate growth and eventually get the plant to cover up that area. You would need to use your judgement to see which of the present branches can be manipulated in this manner. Of course exposure to sunlight would only help to encourage the plant to leaf out. Now if the plant was other than a rosemary, I would recommend further utilizing copper wires to shape branches to cover up the gap, but rosemary stems (at the top) are very soft and also can be brittle so this method may not work out. This method is applied in making bonsai.

Ianna

I have found that Rosemary will not take as heavy a shear as a lot of other Topiaries. You may not be able to keep it as tight and round as other selections.Good luck.