Many coaches and gymnasts are very particular about which guards they want. To some degree it is true that gymnasts can and will adapt to any handguards over a period of time. It is also true that when first wearing guards, it is a big adjustment for gymnasts to make changing from bare hands to grips. In general, it is too expensive and time consuming to try, break-in and test each type of guards, so gymnasts tend to use the guards their friends or coach recommend. There is nothing wrong with doing that.
Usually we recomend for the younger gymnasts to buy first pair of handgaurds which are easy to break in and narrow (Hot Shot style, Pixie, Bailie Narrow guards or Reisport Junior). These guards are very easy to get used to and they allow good hand to bar contact (due to being narrow).
More advanced gymnasts may want to use wider styles like: Millennium, Omega, RKO, Bailie and of course well known Reisport - Swiss Performance.