In this paper, the backward-compatible coding schemes are explored to achieve the near-capacity performances in dynamic LDPC-coded Gaussian multiple access channels (GMAC). Specifically, some user-load-compatible (ULC) masked edges in a raptor-like protograph are masked with the variation of user load <inline-formula><tex-math notation="LaTeX">$u$</tex-math></inline-formula>. To distinguish the edges in a protograph, a multi-user protograph-based extrinsic information transfer (MU-PEXIT) chart is developed. Based on this chart, an iterative edge selection algorithm combined with a theoretical simplification is proposed to facilitate the selection of independent masked edge sets (MESs) for varying <inline-formula><tex-math notation="LaTeX">$u$</tex-math></inline-formula>. To further decrease the complexity of the independent masking scheme, a nested version is presented, where the MESs for varying <inline-formula><tex-math notation="LaTeX">$u$</tex-math></inline-formula> are nested and derived from the same mother MES. Meanwhile, nested selection algorithms are provided to select the nested MESs. Extensive simulation results demonstrate that the masked 5G-NR LDPC codes can maintain near-capacity performances regardless of the variation of <inline-formula><tex-math notation="LaTeX">$u$</tex-math></inline-formula>. Besides, these ULC masked codes achieve comparable performances with the state-of-the-art LDPC codes which are specially re-designed in a static GMAC.