I want to take a gap year and travel after getting my undergraduate degree in Physics. Is it possible to apply for a research grant that would cover my expenses that year? Can someone travel a lot as long as they meet the expectations of a research grant?
What you are describing is possible, however unlikely.
To receive grant funding, you need to work (otherwise youd be stealing). The vast majority of grant funds are tethered to a location and 24/7 remote work is not a reasonable request. This difficulty is compounded by the narrow time window that im perceiving in your question, finding that appointment is a challenge.
The more likely solution here is to make the travel part of your graduate program itself. Apply to a program 'locally', and target a different laboratory of your choice. Then, fully disclose your plans to graduate at the local program, and collaborate with the distant lab. Collaboration could be in the form of materials/facilities usage, etc. For example, summering at a lab for whatever reason. You could even make the case to your new primary adviser that you simply want to learn spanish during your career because it will make you a better scientist (thats a big sell).
This will be a little tricky and will definitely require far more investigating than just applying to a graduate program. But, tethering to the 'primary' graduate program is both possible and productive. this approach relies less on luck.
You cant get around the fact you have to work AND also find that work.
Alternatively, there are plenty of opportunities to pursue research appointments at far off, beautiful destinations (e.g. http://www.bios.edu/#!/who-we-are). These are typically farther down the research spectrum, and will contribute to your research career in a different (i.e. less) fashion. I myself held several appointments previous to grad school, and they were the best experiences of my life. the appointments are much easier to come by. however, i do not consider all of them to be part of my "science career".