Will you have access to a car? If so bear in mind that, unless you time it carefully, there can be something of a queue to get off and then back onto Gib. So try to avoid obvious rush hours and when aircraft are taking off/landing (the road crosses the runway). If without a car frequent buses run from La Linea (just over the birder) to Algeciras where you can change for Tarifa. I think you can also get to other sites by bus (see www.tgcomes.es). The Algeciras-Ronda train also takes you to some good locations.
Naturally, I'd be happy to let you have a copy of my 95 page birding notes/site guide for Cadiz province which others have found useful - send me a PM with your email address & I'll forward a pdf to you. They're free, but if you feel inclined a suitable donation to a wildlife charity of your choice would be appreciated. My website (see below) gives you, I hope, a flavour of the area and a taste of my site guide,