Was the Earth's sea-level significantly lower in ancient times?
There are several questions here, I'm only focusing on the question regarding sea level in the past as per the title question.

In short, global mean sea level (GMSL) has been both lower and higher than present in the past. As recently as the last glacial maximum around 25,000 years ago GMSL was around 120m below present as much of the water was held on land as ice. 

However, GMSL doesn't really exist because sea level is not level, but is a useful way of summarising global trends. What people living in coastal areas experience is relative sea level (RSL), which is defined as the mean level of the sea surface relative to the level of the land. There are two key components affecting RSL: isostasy and eustasy. Put simply, isostasy refers to the vertical position or motion of the land (the Earth's solid crust sits on a molten mantle and is subject to vertical displacement from its equilibrium state governed by the balance of overburden pressures  i.e. a concentration of mass on land causes subsidence of the land beneath it whilst uplifting the land extending beyond the focus of the mass. Eustasy refers to a uniform, global change in sea level caused by fluctuations in the balance between water held on land (usually as ice) and in the oceans, or by a change in the capacity of ocean basin (usually tectonically related). RSL can vary considerably from GMSL (unsurprising since this is a mean measurement). For example (I'll use a British example here since that's where I'm from), at the last glacial maximum ice sheets covered the northern part of the British Isles from Scotland down to around the middle of England  (the exact southern extent is still debated in the scientific literature). The overburden of this ice effectively turned the British Isles into a giant see-saw with the land in northern part subsiding and the southern part uplifting. The effects of this are still felt today long after the ice has melted (and contributed a eustatic sea level component) as the land level readjusts to its equilibrium state( glacio-isostatic adjustment); the land in Scotland is rising and the land in the south of England is subsiding. So, whilst is true that globally sea level is rising  (at a rate of around  3.2 mm/yr), when combined with local land level changes then some places actually currently experience a sea level fall (in Scotland this is up to -1.5mm/yr) whilst in regions of isostaic subsidence the rate of sea level rise can be considerably greater than GMSL. 

Since you mentioned gravity in your question I'll breifly introduce another curious component  (there are others,see reference below) which results in globally  non-uniform sea level changes. We are not experiencing a lower level of gravity  (gravity is a constant).  The amount of gravity that something possesses is proportional to its mass and distance between it and another object. On Earth mass is obviously not evenly distributed, with each location on the planet experiencing a different mass-dependant   perturbation of the gravitational field. So for example, as an ice sheet forms and grows  mass increases which results in a gravitational perturbation which attracts the ocen toawrds the ice sheet but as the ice sheet melts and the gravitational perturbation reduces that water that was previously drawn towards the ice mass is redistributed in distant locations from the ice mass. This (counterintuitively) means that the greatest rates of sea level rise as a consequence of Greenland melt are actually experienced in the Southern Hemisphere.

Hope this serves as a useful introduction. I would usually include more references but I'm writing this on my phone right now whilst travelling on a train. One key reference,which is written in a non-technical style, worth checking out is : 
Sea level is not level : The case for a new approach to predicting UK sea-level rise. / Gehrels, Willem Roland; Long, A.J.
In: Geography Compass, Vol. 93, No. 1, 01.03.2008, p. 11-16.