Tapping a creature on the turn it enters play
It will have summoning sickness; you cannot active its ability this turn.

From the comprehensive rules:


  302.6. A creature's activated ability with the tap symbol or the untap symbol in its activation cost can't be activated unless the creature has been under its controller's control continuously since his or her most recent turn began. A creature can't attack unless it has been under its controller's control continuously since his or her most recent turn began. This rule is informally called the "summoning sickness" rule. Ignore this rule for creatures with haste 


A creature that has the Haste ability can tap immediately, so unless you have a Reflex Sliver on the 'field, you will not be able to tap a Gemhide Sliver for mana on the turn it comes out.