You can, and both platforms mentioned in this guide allow you to hold gilts within a stocks and shares ISA with them.
However, always consider whether gilts are the best use of your ISA allowance. The main point of this guide is to take advantage of the tax-free nature of capital gains on gilts.
If you pay tax on savings interest, then the only advantage of having gilts in your ISA is to save tax on the coupon paid to you. If you've picked low coupon gilt(s), then this isn't likely to save you a lot of cash.
However, if your gilts have a much higher coupon and you've chosen to invest in gilts for the certainty of that return, then the tax gain of holding them in an ISA becomes more significant.
Yet consider whether there are other things you could invest in that means you could make better use of the tax benefits of a stocks & shares ISA - the link has more information about what these are.