glTF 2.0 has an alpha blending mode available, but it's intent is just to be "coverage" (like medical gauze tape), not a physically correct optical translucency (as the latter was considered too high a target for the initial release of 2.0). Basically this means that low alpha values will dissolve the entire material, including specular reflections. Even so, the mode is often used to emulate clear materials like glass.
To control the blend mode, look for a settings panel near the bottom of the material properties set of panels. Blender offers many choices here. glTF supports three of them: Opaque, Alpha Blend, and Alpha Clip (along with the "Clip Threshold" value in that case).
The alpha values can come from an RGBA "Base Color" texture assigned to the Principled BSDF node. If no texture is assigned, you can use the color picker on the Base Color disconnected input itself, and note the color picker there includes an alpha slider.
NOTE: At some point in the Blender 2.8x series, the glTF exporter began using the "Alpha" input on Principled BSDF, rather than the alpha channel of the base color, for textures but not yet for default values. In Blender 2.9x, it will exclusively use the alpha input for both textures and scalar values.
The following Blend Mode must be set (in Eevee, and each material remembers this setting even if you switch back to Cycles) for alpha blending to take effect:
For folks looking for a more advanced, physically plausible optical translucency, take a look at a glTF extension called ADOBE_materials_thin_transparency. Adobe submitted this in support of the translucency available in Adobe Dimension, and Microsoft worked with them to add support to BabylonJS. But, not many other glTF viewers have added support for this yet, outside of Dimension and Babylon.
UPDATE: (July 2020) There's now an effort underway to bring an extension similar to ADOBE_materials_thin_transparency to Khronos as a new extension draft called KHR_materials_transmission. Blender support is under development in PR #1094.