The Gaussian Error Linear Unit, or GELU, is an activation function. The GELU activation function is $x\Phi(x)$, where $\Phi(x)$ the standard Gaussian cumulative distribution function. The GELU nonlinearity weights inputs by their percentile, rather than gates inputs by their sign as in ReLUs ($x\mathbf{1}_{x>0}$). Consequently the GELU can be thought of as a smoother ReLU.
$$\text{GELU}\left(x\right) = x{P}\left(X\leq{x}\right) = x\Phi\left(x\right) = x \cdot \frac{1}{2}\left[1 + \text{erf}(x/\sqrt{2})\right],$$ if $X\sim \mathcal{N}(0,1)$.
One can approximate the GELU with $0.5x\left(1+\tanh\left[\sqrt{2/\pi}\left(x + 0.044715x^{3}\right)\right]\right)$ or $x\sigma\left(1.702x\right),$ but PyTorch's exact implementation is sufficiently fast such that these approximations may be unnecessary. (See also the SiLU $x\sigma(x)$ which was also coined in the paper that introduced the GELU.)
GELUs are used in GPT3, BERT, and most other Transformers.
Source: Gaussian Error Linear Units (GELUs)Paper  Code  Results  Date  Stars 

Task  Papers  Share 

Language Modelling  98  13.41% 
Question Answering  32  4.38% 
Text Classification  25  3.42% 
Sentiment Analysis  22  3.01% 
Text Generation  21  2.87% 
Natural Language Understanding  15  2.05% 
Pretrained Language Models  14  1.92% 
Information Retrieval  12  1.64% 
Named Entity Recognition  10  1.37% 
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