There is too much information on this forum for you use to ask such a basic question. It is unlikely anyone will respond as it is clear as the info is already available.
Not trying to bust you but you need to make a effort to educate yourself using the stickies and search function.
In most cases injections are preferable to gels. Is gramps mobile, can he do his own injections, etc. Is he around grandchildren (with the gel not a good thing). What is the purpose of the TRT?
There are plenty of positive stories about gels and about pinning. Just read some more.