"What should I do regarding shrines and worship if I'm hiding my practice from my parents? (I worship Gaia...might the Earth be a good "shrine"?)"

At some point in our lives, most of us find ourselves in a situation where we are held back from practicing the way we want to. It can be a because we’re living with people who do not understand our religion or practice, or whom we simply do not want to come out to. It can be because we are on holiday, because we have guests over or because we’re busy and life is chaotic.
That having been said, there are a lot of things we can do to practice our religion within the restraints placed upon us by either internal or external factors. Here are some tips to deal with these issues:
  • Strip your religion back to its basics and/or find out which practices matter most to you. This allows you to maximize the time and the amount of privacy you have by uncluttering your head.
  • Make a portable altar/shrine kit. This is a box, can or any other medium in which you place those things you can’t practice without. I have two sets; my original Eclectic one and a Hellenic one. Within my Hellenic box are two tea-lights and a holder, a container with khernips, a container with ethanol, a container with olive oil, a cup for khernips, a cup to burn offerings in, cloth to dry my hands and face, some incense, a hair clip, matches, a little prayer book of the hymns I use most, a spoon and a container of barley. I use this kit when I travel but it can also be used to quickly set up a place of worship and break it down just as quickly. The box can be hidden away when not in use so it does not take up living space, a valued commodity for some people. 
  • Find substitutes. For those who like to have some sort of permanent altar or shrine but don’t have the liberty to do so, find substitutes for the basics of your altar needs. I have seen eclectic altars set up with pebbles, seashells, flowers, pompoms, even Barbies. For those who are not allowed open flame or candles, find substitutes. Electrical candles work just fine and look pretty realistic. Use essential oils to smell and a feather to set the air in motion. Substitutes are not perfect but they get the job done. Often, it’s the thought that counts.
As a final note in regards to the original question: worship on the Earth is generally for Khthonic deities—mostly those of the Underworld. Gaea, however, could be worshipped in that regard as well so yes, you could worship Her ‘on the earth’.